Fanjita has released an application that changes the default speed of the PSP to use the full 333mhz speed on games, heres what the readme says:
This app tries to overclock the v2.0 PSP so that UMD games will run faster.
I know that it doesn't work with GTA:LCS, but you might find something that
it does work for.
Installation
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Follow these steps carefully or it won't work.
- Copy LOADER2.PNG to /psp/photo/loader2/loader2.png, and then view it with
the PSP photo viewer. Set it as wallpaper.
- Put LOADER2.TIF into the same folder. This must be done after you set the
wallpaper.
- Put A.BIN into the root (topmost) folder of your memory stick.
Now open the LOADER2 folder with the PSP photo viewer. You should see a
corrupt data icon for LOADER2.TIF. This means the overclocking is now
active, and your PSP is running at maximum speed.
The overclocking should stay active until power-off, or until some application
resets the CPU speed. Unfortunately, for most UMDs this seems to be when they
are run.
Let me know if you find any UMDs that this does work for.
Download via the comments and let us know if you get any UMDs running faster.