[popcornmix]

I am trying out a new somewhat lightweight skin (this one).
It is running fast and nice…. in the beginning, I noticed that performance gets worse and worse for every minute (small explanation here).

I've not seen this issue. I'm currently running Aeon Nox 5 which surprisingly runs pretty well on the Pi (it's a lot lighter than Aeon Nox 4) and that's been continuously running for some days.

I wouldn't expect it to be PR#4234 - that should only affect going into the top level library views (movies, tv, music), whereas I assume it is continuously sluggish for you?
A guess a memory leak (especially if swap is enabled) could cause a slowdown. As could many other things. Milhouse's bcmstat.sh script is useful for tracking memory/cpu.

Feel free to try to bisect where the error occurred (and push out test builds if others are seeing the issue).

I'd be quite careful about deciding when something is fixed. Make sure you switch between the good and bad build a few times to be sure.
xbmc will be more sluggish while addons are being updated, or have just scheduled something to be done.
Not everything slow that xbmc does shows up in the log (although obviously while it is slow you should check for anything unusual appearing in log).
I couldn't see anything obvious on a quick scan through the log though.