I know that disabled modules do not effect performance. I also know that typically the number of modules you have enabled will effect the speed of you site. One individual on Drupal.org said you should never have more than 40 modules enabled. I found this strange because I would assume some modules are more intensive than others.
Is it simply the number of modules that somehow effect performance, (because the hook is always called), or is it really dependent on what you are doing in the module?
It depends on the module.
You can create a module which has a simple info file and a blank module file, compare that to views which has loads of file and implement lots of hooks and create it's own as well. Obviously this requires more memory than a blank module.
Performance depends on a lot of factors. A module can use a lot of memory but not slow the side down much while the reversed can be true as well. Usually the bottleneck is r/w to the database. A single bad query can take seconds to perform, most tasks in code is not that slow. Iterating over a lot of objects and munching data can, however, take a lot of time.
So it's hard to say how a module will effect performance, but it's always a good idea to limit the amount of modules used when possible. I haven't heard of a thumb rule though and to be able to make one, you would need to know the hosting environment.
