From what I remember, Roblox runs largely off of your CPU, so most of the strain to run these games would fall on the CPU. So, it makes sense that your CPU would be finding itself being heavily used and overworked by Roblox. I don’t know how old your device is but I believe that it is simply not able to handle Roblox’s heavy reliance on the CPU.
Yeah from what I see Roblox doesn’t really seem to tap into the GPU much at all, on my primary device I run a 4070 super and I find that Roblox never uses over 1% of it and uses 20-40% of my CPU at almost any given time. I wish Roblox would make it run more off of the GPU if a device has one but I don’t see them doing it any time in the future.