It appears that Sony is using the PAL version of PS1 classics in its PS Plus subscription.
The subscription has now launched in Asia, but - as spotted by VGC - some PS1 games are using the PAL (Europe, Australia, New Zealand) version instead of the NTSC (Asia, America).
That means they're presented in 50Hz instead of 60Hz, resulting in a lower refresh rate and therefore lower framerates.
However, this isn't the case for all games.
First party titles Ape Escape, Everybody's Golf, Wild Arms, Jumping Flash, and Kurushi are based on the European PAL versions.
For third party titles, both Worms games are PAL, while Tekken 2, Syphon Filter, Abe's Oddysee and Mr. Driller are based on the NTSC version.
As VGC has verified in the tweet below, Ape Escape, Everybody's Golf, Wild Arms, and Kurushi can be seen with English language menus and Sony Computer Entertainment Europe trademarks.
UPDATE: All first-party PSOne games included in the new PS Plus in Taiwan (an NTSC region) appear to be based on the PAL 50hz versions, I've confirmed.https://t.co/hT1Hj048cN pic.twitter.com/t3CxGqv9ua— Andy Robinson (@Andy_VGC) May 23, 2022
There's precedence for this too, as Sony used a mixture of PAL and NTSC versions of games in its PlayStation Classic console.
As Digital Foundry explains: "Most PAL games run at 83.3 per cent of the full-speed of the NTSC versions in terms of frame-rate, and most of the time in terms of actual gameplay speed too. Side-by-side with the NTSC equivalents, PAL versions are generally slow and plodding and don't properly represent a US or Japanese developer's original vision.
"There's little consistency [on the PlayStation Classic], meaning that gameplay suffers from obvious stutter. It's a really, really poor showing here."
By contrast, Nintendo has updated its Switch Online catalogue of N64 games to include both PAL and NTSC versions.
Eurogamer has contacted Sony for comment.