I'm using the printCustomAlbumThumbImage function on my album.php pages to show custom thumbnails, and I sometimes have pages with about 40 or 50 pictures (I prefer them like that, all on one page) and I wonder if using printCustomAlbumThumbImage increases the server load / creation time of the page?

Or is it the same as creating a normal thumbnail, because the custom function just gives a few variable numbers to the i.php script which otherwise would get those variables out of the zp-config, and eitherway it makes the same kind of output-thumbnail with the same width/height/cropheight in the filename.

It's the same as using the default thumbnailing; the slow loading only happens the first time the image is loaded at that size, then it's cached and very fast. You're right about the parameters to i.php -- it makes no difference where they come from.

Unlike other galleries, Zenphoto does all image processing on-the-fly. It would be easy to make a "preprocessor" script though that processes and caches all images at a certain size, but it's just as easy to view the album, which does the same thing.