Eclipse being laggy has a lot to do with what extensions and editors you install. If you know how to configure your setup correctly (ie. not install a huge all-in-one package with tons of crap you will never need, compare different extensions that are similar in functionality and choose for performance), it will be smooth. For me, coding in PHP is a good example - I tried a handful of editors and settled on the dinosaur PHPeclipse instead of some flashy PDT. It has better highlighting (to me, at least - probably only because it's the kind of highlighting I'm used to) and runs fast as a cheetah (I have a shameful couple of files that rank in the 20k+ lines and PDT would stutter on those... when I was moving the cursor). The final issue most people cite is startup time and so far, the only remedy I have found for that is keeping your workspace folder tidy and using an SSD.

To be honest I don't know either. Eclipse has always runned fine until the rare day when some plugin goes overboard and destroys all hope.

Last year for instance I've had bad issues with autocompletion on some huge classes (GLES20 I'm looking at you), but 1) that was when I was struggling on the macbook already, and 2) Someone fixed the problem somehow while I was busy downgrading to a stable version.

Eclipse kicks ass. I only said it was laggy out of mercy for the people who can't get productive with it.