Poor and slow iperf performance

Hi.

Recently i got new server hw with 10GBE which i wanted to performance test before sending it into production.
Up to now i was always using gentoo, because it just gives best network performance out of the box, utilizing full gbit speed in recent tests.

Now i'm pretty disappointed why rhel (7.7) is so slow, its just showing around 500 mbit/s maximum in iperf3 using the same ports on the switch, the same cable, and the same client.
It cant be the hardware, because the gentoo server providing full gbit speed is an outdated (consumer) system from around 2013.
Its a pretty out-of-the-box installation of rhel7.7, nothing out of the ordinary.
If it isn't able to utilize gbit, i don't even want to test 10gbit.
I also tried several iperf TCP window sizes with no good results, it always tops out at around 500mbit/s.

Are there magic switches in rhel that release the handbrake?

cpu and ram are nearly idling when doing that iperf3 tests, so that shouldnt be an issue.