Dungeon Siege - Throne of Agony

[LunaMoo]

From most to least likely - three things could cause that:
- you're using some non-default "best PPSSPP settings!!!1111oneoneoneeleven" aka game breaking settings commonly recommended by potato users on youtube and other social media, most of the time forcing emulated PSP cpu clock(which should always be set on auto - let's the game decide) is used as speedhack and that would cause a drop in framerate at best, at worst causing crashes or other bugs, often both,
- the game actually runs at 20 fps - most PSP games run at 30, but less is definitely more common than more and 20/20 was nothing weird to see,
- an emulation issue - while rare, there are games which runs at incorrect FPS due to timing issues and the fact that PPSSPP is much faster still than the real hardware, more commonly it would run at higher fps than designed through.