PSP » Strange game speed issue with Gran Turismo 2 on PSP?

I thought all PS1 games can be played at full speed on PSP.
Until I tried Gran Turismo 2 NTSC-Japan version, NTSC-US version and PAL-Europe version on my PSP 2000 (CFW 6.60 Pro C).

The game speed of two NTSC versions are a bit slower(Only slightly). I checked with the timer of the game with a stopwatch. When in the game the timer has passed 10 seconds, actually it has passed around 15 seconds.

But PAL version is running at full speed. Maybe most people didn't notice this because the NTSC version is still fully playable.

Please Help.

Thanks !

I didn't find the right solution from the internet.

References:
http://www.neoseeker.cums/23172/t2271211...n-turis-on-psp/

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