Hi,
I purchased Asset Forge Deluxe recently, and I'm having the exact same issue. The program is running fine, and is very responsive, but if I do something else while it's open and I come back to it, it becomes so slow that it's unusable, using 100% of my GPU, this is very frustrating. When it's acting like that, the only thing to do is sometimes to reboot the computer, even force quitting the program and launching it back doesn't do the trick.
I'm on Windows 10, and I tried enabling V-Sync, this doesn't change anything.