Only roblox lags on my pc, every other game runs fine

is it a laptop? check roblox is using the graphics card and not the integrated chip. windows settings > system > display > graphics, add the roblox player and set it to high performance. other games tend to pick the right gpu on their own, roblox often doesn't.

roblox leans really hard on one cpu core, so a pc that runs bigger games fine can still choke in a busy roblox server. set graphics quality to manual and drop it a few notches instead of leaving it on automatic, close chrome while you play, and if it's been like this a while a clean reinstall fixes more than you'd think. also check if it's actual low fps or ping spikes, a lot of roblox 'lag' is the server, not your pc.