I guess that's why I am not maintaining Linux distribution mainly targeting real hardware. Support of hardware is a real nightmare. Linux forums are full of it.
Testing with by wire, if that's possible and see if it still happens would also be great to exclude possibility that Whonix itself is the cause.
Whonix-Gateway's firewall hasn't changed a lot during these versions. Please feel free to see the diff.
git diff 7 whonix_gateway/usr/bin/whonix_firewall
I couldn't reproduce the issue the other user described and there are no other reports. The user didn't reply to my follow up questions yet. Depending on these answers, I hope it's a non-Whonix caused issue. Otherwise it would be really difficult to track, since I never experienced it myself.
As for debugging Whonix's firewall, you could also completely disable it. Then you'd still be able to access ports Tor is providing (such as 192.168.0.10:9152 ... 9153 etc. [https://www.whonix.org/wiki/Stream_Isolation]). (Transparent Proxying would be disabled then.) And then do Tor benchmarks.
Sdwdate could be a cause for Tor issues. It changes time and Tor may not cope up with that well. Sdwdate doesn't run often on Whonix-Gateway, though. Sdwdate can be shut down (sudo service sdwdate stop) or you can watch when it runs (tail -f /var/log/sdwdate.log).
I might come up with more if needed.