I had to change the single-fotopage ending in zenphoto from .php to .html due to restrictions in apache's modsecurity. The rules won't allow combinations of jpg and php which could be used for fake-sites obviously.

Interesting is now: All the single fotopages seem to load faster, switches from one to the next is more smooth.

Welcome then..;-) Interesting observation. I could imagine being no server expert that maybe the modsecurity acts more relaxed on the html suffix and therefore the server delivers faster.