As far as I know, the only way to decrypt an ISO is by using the PSP itself, so yeah.
But, you don't need to decrypt the whole ISO, only the clean/untouched EBOOT file that can be found inside the ISO.
Ask someone who own a PSP to do it for you, then you can compile the ISO by yourself using the UMDGen.
However, since you get a black screen and zero fps running, it could probably means that the ISO you trying to load is either patched or
has prometheus loader in it, because if it was decrypted, a message saying the ISO is encrypted should be appear just after you load it.
and use JPCSP
, as it show more progression, updates, and able to run more games, than PCSP.