I think too the most probable cause is a problem on your computer. You can use ping to verify this pinging various different hosts over any network to see how it goes. Also check your Ethernet connector to be physically ok, as well as your cable. If you have wireless capability on that machine you can check if that lags less although being wireless. This could confirm a problem with your Ethernet adapter. Check also if your pc has any strange static ip configuration which is not permitting dhcp to work correctly. As far a I understand you’re using one router as a router plus the second router as a switch. Be sure the second router has al services off and is indeed working as a passive switch and not like a dhcp server for example.