This was reported for the mt7530 switch driver tho. Should only affect the AX6S as AX3600 uses QCA switch? Btw, when I studied the mt7530 driver I find that the VLANs configuration code is not ideal, but it shouldn't cause thruput throttling. The one that I found causes issue for the mt7530 switch is the disabling of the source address learning capability of the switch. When I disabled the 'disabling of SA learning' the mt7530 router (in my case the Linksys E8450) goes back to full speed even when 10/100mbps clients connected. The entire patch that was reverted to 'fix' the issue can be retained.
Edit: I mistakenly thought that this is also a thread for the Xiami AX3600, but it was for the AX3200, which is mt7622 based, using the mt7531 switch. In any case, the above point still stands tho.
Interestingly in the R7800 thread it was also reported that if there's a 100mbps client connected to it and there's active data transfer between that client and other LAN clients, WAN thruput will be affected. I did a quick test with my R7800 and I also see the behaviour as reported. It doesn't look like it could be caused by the switch driver, but I do not have enough knowledge to determine the cause. My suspicion lies in the way data frames are passed between the GMAC and the CPU. Likely the same issue is affecting the AX6S and AX3600 and likely all routers?