Thanks for your prompt answer.
And sorry for mine being (too) long.
Well I'm just looking for a way for the cache to go faster, as I'm convinced that the NAS is able to. The provided gallery (called PhotoStation), although not so customizable, was creating thumbnails and caches much more faster.
But if I quit using it, it's because I wanted the flexibility ZenPhoto offers.
Except mistake from my part, I was able to activate ImageMagick on Gallery3, which is also written in PHP. But I do not want to enter this debate. I tried g3 and did not like it. They might have implemented it in a way you're not willing to reproduce.
My difficulty is that I use zenphoto together with a set of lightroom publishing service: each time I modify some photos, it is updated to the NAS. Last time something like 800 photos were refreshed, of which 600 were in the same album. And every time I have to log into the admin to refresh ImageCache, and it takes a very very very long time (several hours in that particular case).
If I do not do that, zenphoto will be very slow to display some album pages where pictures were updated, due to the on-the-fly caching. And it can even go in timeout (in the 600 photos album case).
This is why I'm looking for a way to improve that process on the one hand, or to trigger it by night on the other hand.
For HTML cache, I already set it to 1 day and I placed a cron script to refresh it every day as well (in case of a new album for example). And I'm satisfied with that.
For image cache, I still have to trigger it manually if I want to avoid my visitors a long and uncomfortable wait.
I probably could upgrade my Synology, but I'm overall satisfied with it and I'm sure it could handle the task satisfyingly. And as noticed in the forum that you finally have a lot of synology users (normal, this box is awesome).
Maybe I'll have a look to how GD and Imagick libraries are implemented in zenphoto, to see how far it could be transposed to command line calls. Not to replace the current one, but to offer an alternative. However I doubt I'll find the courage to do it
Thanks anyway!