Unless FGU is running your GPU at high clocks and Foundry is running it at low clocks those 90%+ GPU load readings look bad. If using dedicated GPU measurement software and/or driver updates don't solve that then running FGU surely is better suited for your system.
Concerning memory comparisons we would need to know what you load in each system. But again, in the end your best pick is to use the VTT that runs most smoothly on your computer *and* the computers of your players.
I am currently running Starfinder on Foundry using maps with several dozens of animated lights and none of my players even mentioned performance issues. So performance is not even something I have to discuss with that particular group (no potato computers). Instead my main concern with that group would be that I cannot buy ready-made content for SF on Foundry and thus am forced to create walls + lights on the Paizo PDF imported maps. The results look very nice, but it takes my time to prepare them. On FGU I can just buy the stuff and then have to live with the facts that I cannot even make it as nice looking even if I wanted.
After a short test-run it is very likely that the group will start a Pathfinder 2 "Strength of a Thousand" adventure path, probably switching to at-the-table playing later. At this point the main discussion point will be whether they are willing to install FGU software and create accounts on top of having to learn another VTT, because for PF2 I get finished maps on both VTTs (free on Foundry). As a GM I now have the freedom to offer both VTTs, even with my reluctance to put more money into Smiteworks for the time being.

