Damn...

[[Unknown]]

Generally, even a not very recent PC should be able to run Gran Turismo with reasonable settings just fine.

Can you try the latest git? It can be found here:
https://buildbot.orphis.net/ppsspp/

There were some effects that were ignored / not drawn properly in older versions that that game used, so depending on how old it was this might be involved.

It may also help to upgrade graphics drivers or change the graphics backend setting. If you haven't played in a little while, your drivers may have been updated since, and could be causing the problem. Compared to emulators for other systems, PPSSPP's emulation of the CPU is very fast - so usually performance problems are graphics related.

-[Unknown]