Ever since updating to plasma (5.3), I'm having performance problems with my nvidia 260GTX.
The animations are not fluid and stutter from time to time. The splash screen is the main offender when it comes to animations.

There is also an impact on applications like mpv: I can no longer run it at my usual settings, it now stutters when playing back the same sort of videos it used to play back well. Switching to e.g. openbox solves the problem.

Reproducible: Always

Steps to Reproduce:
1. enable kwin
2. GLX backend + any openGL version
3. vsync: anything except "never"

Actual Results:  
choppy animations
mpv video playback choppy

Expected Results:  
fluid animation
fluid mpv video playback

The "not a benchmark" fps effect shows red spikes whenever a video is playing and stuttering. I can't tell the fps it's showing, because the display looks like this [1] with any text color and position.

It definitely used to work before on KDE4. Disabling kwin is not really an option, because that kills vsync and one particular feature (smooth motion/frame interpolation) of mpv requires it to be active.