Fedora 33 Ethernet speeds low with new kernel, a little better with old

I have done a series of tests with different machines on my LAN (all ethernet) using iperf. For some reason, the most recent 5.10 kernel tops out at 351 Mb/s. The OLD kernel 5.8 (in the grub menu) gets double that at about 768 Mb/s. Now here’s the thing … if I go between two debian/ubuntu machines I get a full 931 Mb/s which is about what I expect. There is no setting that’s wrong and it’s obviously related to the kernel. Has anyone had this problem? Is there way to tune the kernel and make the network throughput what it should be?