Low bandwidth between 2 servers connected to the switch via a 20Gb link

Hi, maybe someone can help me?

I have this infrastructure:

2x dell's PC8024F switches stacked with 4 fiber ports, 10Gb/s each.

1x Windows Server 2008 R2 on Dell R520, with network card with 2x 10Gb, each port connected to one switch, ports on switch are in LAG, on windows in team, connection on windows is raported as 20Gb/s

1x Windows Server 2012 on Dell R620, with network card with 2x 10Gb, each port connected to one switch, ports on switch are in LAG, on windows in team, connection on windows is raported as 20Gb/s

On all switch's ports used to stack and used by windows i have jumbo frame set as 9014, and the same on windows's network cards.

I checked bandwith between this windows servers using iperf3 tool, and i have only about 2.7Gb/s. 

What I can change in configuration, to get throughputs at least 10Gb??

Best regards,

Mariusz