Can you check the link speed between the client computer and the router? If the client isn't close enough to the AP or it is using the 2.4Ghz band instead of the 5Ghz then the link speed might be the limiting factor.
I have a AC87U running Asuswrt-Merlin 376.48_1 that is running dual-wan in load balancing mode.
Since using dual-wan webpages seem to lag in that chrome says "Waiting for (some domain)" and in particular HTTPS sites seem to have the problem more often in that the page only partially loads...