Why does a Quad core not perform better then a duo core?

The update installation will be drive-limited -- not CPU limited.

For applications that use the extra cores, the quad core will be faster -- for others, not.

To expand on ejn63's correct answer, the time to install a Windows update is a terrible benchmark for ANY purpose, not just CPU performance. There are far too many variables across Windows installations for that to be useful.  For example, some Windows services might be taking a while to stop in the background before some part of the update can proceed, or you might have some application that's interfering (anti-virus, for example).  And yes, storage would be the limiting factor on an update installation, not CPU, but even with identical hardware, one drive might be more fragmented than another (not an issue with SSDs, but you get the idea), or one SSD might have completed TRIM more recently and therefore have faster write performance to its free space.

However, even if you were looking at an activity where CPU performance was the main factor, a quad-core CPU will not always be faster than a dual-core CPU.  The newly launched 8th gen Intel Core CPUs are a perfect example.  They've added quad-core i3 processors for the first time, but the clock speeds of the individual cores has been reduced significantly.  As a result, for workloads that can actually take advantage of more cores (or PCs that have enough independent tasks running at once to keep them all busy), the new quad-core i3 will be faster OVERALL.  However, if you were looking at the performance of a specific task that did NOT use multiple threads efficiently, it's possible that it would be SLOWER on that new quad-core CPU than an older dual-core CPU that had higher clock speeds on the individual cores.  Gamers look at this very carefully because many games don't make great use of multiple cores, so they often see better performance with fewer cores and high clock speeds rather than lots of cores with lower clock speeds.