Castlevania SotN: PS1 ROM vs PSP version unlocked in Dracula X Chronicles

Hello

Can anyone explain or better, advise a fix for the following behaviour on my Galaxy S4:

Castlevania Symphony of the Night PS1 ROM plays great in PCSX ReARMed.

Castlevania Dracula X Chronicles also plays great in PPSSPP, however once I load up the Castlevania SotN version unlocked from within Dracula X, PPSSPP seems to struggle; I get crazy slow-down, to the point of it being unplayable.

Have tried disabling both vsync and threaded video driver but no change.

Any help appreciated.