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Sony is apparently trying to remain incredibly faithful to the original PlayStation 2 releases of the games they recently put on the PlayStation 4. In the UK, the emulated PS2 releases are again running at 50Hz and converted to a 60Hz output. This creates some weird frame-blending effects that look bad to the player and result in "frame-pacing issues" that resemble micro-stutters or hitching when the framerate is actually perfectly fine. It's quite easy to see the effect in the video below.
Digital Foundry noticed this in a number of games they recently tested.
Considering you're spending upwards of $15 (USD) for these games, you should expect that they wouldn't be a poor port. People are already upset that Sony dropped the ball on "backwards compatibility" with this move, the least they could do is offer up a native 60Hz experience for all PlayStation 4 owners.
Digital Foundry noticed this in a number of games they recently tested.
UPDATE 8/12/15 4:52pm: We've been looking at more PlayStation 2 titles running under emulation on PlayStation 4, and it now seems clear that all the UK code we've tested is running at 50Hz with a crude frame-blending 'upscale' to the 60Hz output. We made an error in the original analysis below - GTA San Andreas under emulation does have some frame-pacing issues, but much of the judder we encountered is actually a result of the frame-blending, registering to our eyes (and our analysis tools) as unique frames when in fact they are simply the interpolated results of two images - we did think that it may be an artefact of the emulator working with PS2's original motion blur effect, but this is definitely not the case. Credit goes to commenter Malek86 for noting that his copy of Twisted Metal Black is definitely European code, albeit outputting at 60Hz - a situation we have also confirmed with Dark Cloud. This was originally a 60fps game in NTSC territories, running at 50fps on the PS4 emulator, then frame-blended back up to 60fps. As you may imagine, this is not ideal.
Considering you're spending upwards of $15 (USD) for these games, you should expect that they wouldn't be a poor port. People are already upset that Sony dropped the ball on "backwards compatibility" with this move, the least they could do is offer up a native 60Hz experience for all PlayStation 4 owners.