Thanks for the response flop25,

Yes it is very slow and occasionally rather quick when it wants to be, though the vast majority of the time it<s painfully slow. The website would normally slow down with multiple users on it (such as after a large event is put online), but it stays very slow now even when used late at night. Do you suspect that the server may simply be overloaded with images?

Thanks for the advice, I'll get in touch with the guy managing the server.

I you are not lucky, a few or just one corrupted entry in a mysql table is enough to cause this kind of problems, but I suppose you already tried to repair/optimize your piwigo tables, as there is an option in your piwigo administration > tools > maintenance menu ?
If that's not enough and your gallery is linked to other databases, you can also repair all your tables with phpMyAdmin, most hosting services provide phpmyadmin (through cPanel for example), even shared hosting services. I believe the piwigo tools are secure, but be careful with phpmyadmin if you are not used to it, as it's a very powerful tool and allows you to erase and delete everything in you databases with one click ! just be sure to only repair and optimize your tables ! backup/export everything is also a good idea before modifications.