Hi Christoph, sorry about this (everyone)
I just pushed a hopeful fix to trunk in this commit (svn r76942), if it seems ok then i'll backport it to 21.x and 18.x as both have this problem quite badly.
It will be available for testing on the "dogfood servers" soon - e.g. https://nextdev.tiki.org (basic login with next and next).
There are probably a few wishes on the bug list too, but i'm done for the day...