XPS 9300 reducing speed when battery drops below 15%?

It's very possible - but you shouldn't routinely run a battery that low anyway.  Doing so will appreciably shorten its lifespan - and likely hasten the swelling of the battery that happens to many of these at end of life.

The three surest ways to shorten the life of a lithium ion or polymer battery:

Overheating

Overcharging

Deep-discharging.

That doesn't apply if the battery is managed by firmware. When the battery 'runs out' it hasn't actually run out, it's just the firmware saying so. So you can absolutely use your notebook battery until it 1%. Overcharging and overheating are impossible for the same reasons.

Please don't let your battery go below 15 %

At 15 % or below, turn it off until it's plugged and charged (don't disconnect before 80 %, dont let it run all the time plugged once at 100 %)

Guys you are living in the past and dealing out incorrect advice. On an ultrabook 15% can be many hours so it'd be ridiculous not to use the battery's full potential.

Can someone from Dell confirm whether there's CPU undervolting going on and if there's a way around it?

It's unlikely.  More likely, as the battery nears empty, its voltage is decreasing, which is why the system slows down.

Lithium ion batteries have remarkably flat voltage output over time, but EVERY battery, no matter the chemistry, will see a voltage drop as it nears empty.

@fiskjavel  It's true that overcharging is prevented by firmware.  But it's also still true that running a battery near empty shortens its lifespan.  Yes, on modern ultrabooks 15% remaining life can mean a lot more usable TIME than it has in the past -- but that doesn't mean that the chemistry of the lithium-ion batteries has changed.  And if 15% means hours of usable time on your laptop, it also means that the 85% you already consumed was a huge amount of time, so you could (and SHOULD) have charged your battery up again long before you ever got down to 15%.  Obviously it's your PC so you can use it however you prefer, but don't delude yourself into thinking that this thinking is all from the past.

The major things that wear out lithium-ion batteries are being actively discharged -- especially deeply discharged -- and also being kept topped up at max capacity all the time.  That last point is why for example Tesla vehicles limit themselves to an 80% charge unless you specifically specify that you want a Max Range Charge, which you have to specify every time you want it; you can't make that the default.  And it's why Dell and many other laptop vendors offer BIOS settings allowing you to limit your max charge capacity.  iOS has recently implemented a routine whereby when you plug your phone/tablet in for charging overnight, the phone will only charge itself to 80% initially and sit there for most of the night, and it and will wait until shortly before it expects you to unplug it in the morning (based on machine learning, checking your calendar/alarm, etc.) before charging all the way up to 100%.

I have my own laptop set to never charge past 80%, and then I also set a MINIMUM charge level of 50%.  This means that once my battery reaches 80%, when the system is running on AC power, it does NOT keep the battery topped up.  It doesn't actively drain it either since it's still using AC power to run, but instead the battery is allowed to "self-discharge" down to 50%, which will take several days, and only THEN will the firmware charge the battery back up to 80%.  Since my laptop spends the vast majority of its time plugged in, this means that my battery spends the vast majority of its time completely idle, i.e. not being charged, nor actively discharged, nor being kept topped up.  That's the best possible setup for longevity.  Of course the possible inconvenience is that if you have to disconnect your system from AC power unexpectedly, you might only have a 50% charge available.

@fiskjavel  one additional note: With respect to the firmware just saying the battery has run out even when it hasn't, the nature of battery controllers is that the estimated remaining capacity is always an estimate, and it gets less precise as the battery drains.  The reason is that as ejn63 already said, lithium-ion batteries maintain a remarkably consistent voltage until their capacity reaches very near the end, then the voltage drops off a cliff.  Some other battery chemistries have a voltage that drops off linearly with capacity.  The problem this creates is that it's difficult to predict exactly how much capacity a lithium-ion battery has left.  This is why you might have seen your device keep operating at a reported 1% capacity for far longer than it "should" have based on the estimated drain rate up until that point.  It's also why laptops can die unexpectedly when the battery was reported to have 5-10% capacity remaining.  In both cases, the battery controller estimated remaining capacity incorrectly.  Smart battery controllers will use full discharge events like this to revise their estimates going forward, so in that sense they can be helpful, but they also do reduce longevity.

The advice shared here is not incorrect advice based on people living in the past.  It's still completely current and correct advice that is rooted in the chemistry of lithium-ion batteries.  Laptops are still using lithium-ion batteries, and there have not been any major changes to their chemistry that change these fundamental principles.

Fair enough - but it is still not an answer to my question: how do I keep operating at full speed. I bought this laptop for its presumed long battery life. I often work full working days away from home. I need 8 hrs and I need them at full speed. Saying that I shouldn't use my laptop once the battery drops below 15% isn't addressing the problem, much less helping me.

Apart from the solutions presented by other members over here, I would like to add...

To plug in the laptop to a socket and charge the battery while working.

This will ensure that you can discharge your tasks on the laptop and it would charge itself as well.

@fiskjavel  Go to Start > Settings > System > Battery and make sure the "Turn battery saver on automatically if my battery falls below" setting is disabled.  Battery Saver reduces performance -- but if you turn it off, obviously expect reduced battery life.

@fiskjavel  Well given that the vast majority of laptops that Dell sells come with Windows -- in fact I think the XPS 13 might be the only laptop that's even available from the factory with a Linux option -- I don't really think that was an unreasonable assumption to make, even if it happened to be incorrect in this particular case.  Maybe you should have specified your OS in the first place if you didn't want people to make assumptions or at least just said at this point, "I'm sorry, I should have clarified I was running Linux," rather than coming back with a "You're assuming I'm on Windows" attitude toward someone who's spending their time on a volunteer basis trying to help a random person on the Internet.

If you've ruled out the possibility that this behavior is triggered by something in your OS, then if you also don't find any settings in your BIOS Setup for customizing this behavior, then even if it is in fact firmware-controlled, it might simply not be user-configurable behavior.  For example, the XPS 13 9350 and 9360 had PCIe x4 NVMe interfaces, but they were configured to run them in GT2 low power mode rather than GT4 high performance mode.  As a result, performance was roughly equivalent to a PCIe x2 interface, capped at roughly 1.8 GB/s.  GT2 vs. GT4 behavior is technically a firmware setting, but Dell never offered a way for users to change it.  So if you don't already see something promising in your BIOS Setup to manage this, then based on past experience I wouldn't hold out hope for an option like this to appear via firmware update later, in which case your options would be to live with it or get a different system.  Or get that power bank I mentioned as a workaround.  If you're running your battery that low on a regular basis even while it's brand new, then unless your usage patterns change later, this situation is only going to get worse as the battery's maximum charge capacity drops with time and usage -- and that drop will happen more quickly if you continue discharging the battery that deeply.

You're assuming I'm on Windows. This is a Linux laptop. I've not configured it to slow down at 15% nor do I know of any settings in the OS that would make it do so. Ergo, it's probably a firmware setting. The question is (and remains): how do I make that not happen?

Feels a bit like the story with Apple slowing down its iPhones after a few years to maintain battery life. That may be helpful to some users, but, without a setting, it's feels more like quackery to me.

This is why I'm on this board, hoping to get an official response from Dell to rule out or in any chances of making things work the way they should. And also to serve as a warning to potential buyers who might not like the idea of reduced performance just because they use their laptops away from the socket for more than 5 hrs.

@fiskjavel  I should add that if you are regularly running your battery that low, and therefore disabling automatic Battery Saver might reduce your usable life to the point that the lifespan is no longer acceptable, you might want to consider getting a Dell Notebook Power Bank Plus (link to item in US store).  It's basically a power bank with enough wattage output capacity to run a laptop, which most power banks don't offer.  Specifically, it can act as a 65W power adapter, whereas the XPS 13 only requires 45W.  And its energy capacity is 65 WHr.  For comparison, your XPS 13's internal battery is 52 WHr, so this power bank would more than double your capacity -- and even if you don't need that much capacity, using them together would prevent you from draining either of these batteries near empty, which will preserve the longevity of both of them.  And the Power Bank Plus supports being charged in-line with a system, so you can still use a single laptop wall charger to charge this power bank and the laptop together.

@fiskjavel  I gave you a workaround in the form of the power bank, which as I said you're likely to need anyway as your battery loses its max charge capacity if you're already running it down to 15%.  I agree that if this "automatic performance throttling on low battery" behavior is caused by firmware and there's no way to disable it, then that is disappointing, and this would be the only case I'm aware of where that type of behavior was implemented and forced.  But that last part of my previous sentence is exactly why I'm skeptical that that is the situation here.  But since I don't have an XPS 13 9300 in front of me, nor do I have any meaningful experience with Linux, I can't poke around the BIOS Setup myself or suggest what to look for in your OS for ways to control this behavior.  Best of luck with other replies.