tl;dr -- Netbeans is still the best option on a slower machine.
The really good alternatives, like Zend Studio and Aptana, are even slower -- they're absolutely wonderful to work with, library-aware, have integrated everything and play nice with version control systems like Mercurial, Git and SVN... and they're based on Eclipse, so you need a gronking supercharged workstation-class machine to keep from feeling like you're working with a VT-100 and waiting for your keystrokes to be echoed back from the mainframe. At least with Netbeans, once you pay the start-up tax you're pretty much up to working speed on an older machine.
You can get faster IDEs than Netbeans, but I haven't found a single one that's anything like satisfactory. If you choose a font where you can actually tell the difference between 0 and O or 1 and l, or m and rn, then the on-screen cursor is usually off by a couple of characters and selection marking is a joke. There are really good, fast editors, but things like debugging are all outboard, so you're not really gaining that much.
If you're on a *nix machine (Linux, BSD, OS X) you can make Emacs do the job if you're into keyboard chords -- most of the Lisp has already been done and can be downloaded; you just have to accommodate what other people thought were reasonable key combinations. (You'll probably wind up thinking that neckbeards have oddly-shaped hands.) It's fast, it does the job, but it's a real pain in the butt to set up and you have to memorize everything -- it's like working with the old DOS version of WordPerfect (ever seen one of those plastic keyboard templates?).
