i know this thread hasnt been updated in a while, but im also experiencing the same issues as the original poster.
i went form a SH3 in Router mode connected (physically) to x2 cisco meraki AP's and everything was running ok for about 1 year (I'm on the 350mb oomph VM service btw).
I decided to upgrade to wifi 6 using cheap huawei AX3 routers... one ax3 to replace my SH3 as a router as well as doing wifi6, and the other ax3 just for wifi 6 downstairs. i've been meaning to stop using the SH3 as a router for some time as i've had issues with port forwarding as well as the thing timing out on pretty much any setting change (the http apache running on it is slower than any device i've ever used).
after moving everything across to the new AX3's and turning the sh3 into modem mode, it was all ok, but 2 days laters, i started my QNAP downloading torrents and it pretty much killed the SH3. all downloads (torrent or not) were resticted to 100kb/s ish for all clients on my network including my poor wife on work calls at the time.
i had to reboot the SH3 to restore normality.
tried again and low and behold it done the same thing.. speedtest.net went from 380mb/s to barely 1mbit the instant torrents kicked off.
temporarily i've reduced the no. of connections on my torrent client as well as disabling all the other protocols (DHP/LSD/PMP etc). This has helped a lot. though i fear its not the fix and im going to have to turn the SH3 back into router mode soon to get things up and running again possibly with double NAT (gah).
roll on the SH5 (though i doubt virgin will send me one on the 350mb service 😞