I use Zend which uses Eclipse as a base. About how many files we talk?
I use it to manage a complete ecommerce shop with several thousand files (basically because the products have there own html template to show etc) and never got problems.
Maybe try to increase memmory etc?
I'm gonna try out netbeans; I have used Zend quite a bit, but it has 1000 different "perspectives" and I only need 2 or 3. It's often a pain to set up new projects if they are not set up in exactly the same environment as the ones before.
Notepad++ is pretty good, too, you can get a lot of the same features as Zend if you install some plugins.
I'm not sure how people write code without it.
I mean, I guess I can strain myself real hard, and remember far, far back to a time before ^w and ci" or dt; , when my editor was bound to my desktop instead of inside an instance of screen on my VPS, when "G" would enter "G" into the buffer instead of scrolling to the end of document, and clumsy point-and-click GUIs try to emulate good regex support but... *shudders* giving me the willies just thinking about it.
I used to use Zend but have gotten accustomed to dreamweaver recently and like it. Although not sure if it is a true "IDE", I haven't done anything too intense recently so haven't really needed it.