It totally is a glorified text editor + some support tools and I'm pretty sure the reason it's eating that CPU is because of piles of unoptimized cruft. Like every other big project in contemporary "it's nil until you ship it" culture.
Keep in mind that JetBrains IDEs started not as a text editor, but as a rename utility.
So really, it's closer to an AST tool with text-editing features.
Editor is definitely the core but all the support tools make a big chunk of features and usage. Sure, I can get quite a lot of those features in Sublime with some packages but they aren't that well integrated and easy to use!
That said, consider Microsoft Word. Since forever, it's been doing similar - if not greater - amount of work processing and reprocessing text every keystroke than IDEs contemporary to it. Yet somehow, it is not bloated, it does - and always did - work well on even low-tier machines. Also compare with OpenOffice, which does more-less the same things, only much slower and buggier. If M$ can keep Word functional and performant, I'm pretty sure Eclipse and IntelliJ could be made fast as well.
P.S. as someone who has commit access to LibreOffice (and much to everyone else's chagrin, sometimes uses it), I could be a little bit biased.
OTOH, all Java IDEs I've used so far lag by default. It's often about the little things - like those 1 second delays in menus, or 100ms delays between keystroke and letter appearing on screen, etc. Enough to make the experience frustrating. If I keep using them it's only because some advanced features don't have an equivalent (or easily installable equivalent) in Emacs. But it's a tradeoff between having a powerfool tool that I use every once in a while vs. enduring the constant experience of bloat.
Features of IDEs are cool, but they seriously need to optimize them. And to stop assuming that every developer gets to work on a brand-new machine. Maybe it's true in the US, but it's not true in other places (sometimes because managers don't see a problem with paying developers $lot but then being cheap on the equipment those same developers have to work on).
I personally use LibreOffice quite a bit, but something about the name sounds silly to me, so I avoid speaking it. I usually refer generically to "a word processor" instead.