Ppsspp and my device resolution

[TkSilver]

If you are using the buffered reaolution setting in PPSSPP (and you should unless the game crashes with it on.... ie very rare exceptions) then you have 3 resolutions to really worry about. (Note this assumes an android "device" since you mentioned device)
PPSSPP options
Rendering Resolution -- PSP internal resolution the is the resolution that everything on your "virtual PSP" is calculated based on (this is closer to changing a resolution inside of a game). Raising this will have the most inpact on how fast the emulator runs.

Display resolution (hw Scaler) -- this is the resolution of the images sent to your screen. If this was a pc this would be the windowed resolution or if you select (native) then that would be like full screen. However since android does not really do "windowed mode" it then gets adjusted by your device to the screen's resolution (much like how if you watch a 480p video in youtube it gets streched to be fullscreen if your device has a higher resolution) changing this has an effect on performance but not as drastic as rendering resolution.

Physical device
Screen resolution -- this is the resolution of your device and for the most part it is unchangable (there ar some root tools and a samsung program that can, but testing it on my 2560x1440 did not improve performance in PPSSPP since I could set display resolution seprate already) performance gains vs effort .... depends on emulator and on the device, but questionable.

Some side info
1x psp = 480 x 272
2x psp = 960 x 544
3x psp = 1440 x 816
4x psp = 1920 x 1088
5x psp = 2400 x 1360