Yes it is.
We see this constantly in ROOter with MT7621 routers and now in the MT7981 as well.
ROOter use the completely standard OpenWrt USB packages without any changes yet we have 5G modems that top out at 250-300 Mbps with MT7621 and 500 Mbps with the MT7981. This is with modems and connections that should be producing over 800 Mbps.
We seem to have tracked this down to the USB system as the same modem and connection in a M.2 to Ethernet adapter will give the higher speeds. Put it in the USB based M.2 interface and you get the lower speeds.
The other thing that we see is that the factory firmware will give the higher speeds as well on different routers using the MT7621 or MT7981 chips set. These firmware are mostly based on a hacked up version of 17.01 with the MT7981 using 21.02. In some cases they will give speeds twice as fast.
I have been told that 19.07 will give the higher speeds but have yet to have that confirmed. It appears that something in the USB system was changed after 17.01 (or possibly 19.07) but none of the people that are involved in tracking this down have a clue what it is.
But it is a real problem that appears on different chip sets when high speed 5G modems that use the USB system are present.