Archer C7 Poor performance of 100mbps ethernet

When i connect my laptop to the lan port of router(Archer C7) via 1gbps ethernet everything works fine.

When i connect same laptop via 100mbps ethernet weird things start to happen:
-speed between this laptop and computer connected to other lan port of router is as expected, ~95mbps
-but speed between laptop and wan is not so good-17mbps.

Out of the box it worked fine, problem started when i installed relayd.
Also, dd-wrt suffers from this problem out of the box and that's why i moved to openwrt

By the way, problem doesn't exist on wifi, also MTU is set correctly everywhere

I was able to kinda fix this problem-installed Smart Queue Management and set 100mbps limit on eth0.
After i did this, speedtest shows my full internet speed-53mbps. If i change SQM limit to 200mbps, speedtest drops to 17mbps again. But i guess there is still some performance hit and i won't get full 100mbps.

But why is this and what's the proper solution?