Whether To Code In A Form Module Or A Standard Module?

I'm wondering how other members here make decisions whether they want to place codes behind form or use a standard module instead.

I understand there is a performance penalty when you add another module (and use it), but am not sure whether one big fat module would be faster than several smaller modules with identical coding.

Furthermore, I know that some members use a hidden form to deal with startup and shutdown processing. Sometimes the processing has nothing to do with forms and would make more sense in a standard module, but since the form is already loaded, does it makes more sense to use the module behind the form than calling a function in a separate standard module to execute the needed code?

So, what do you tend to do in such situation?