I plugged my laptop directly into one of the LAN ports and ran speed tests, which showed better upload speeds of 500-600 Mbps. However, running the speed tests directly on the router reported lower speeds for some reason. I was following another post on this forum GL-MT6000 Slow Upload speed, which led me to think that running the speed test from the router, being "closest" to the ISP modem, should report the best results. In the end, it seems that the endpoint connected actually matters more.