Honestly IDEs in general are the most complex pieces of software that I frequently encounter. Most of that complexity comes in the form of features that I either never intend on using or don't intend on using at the time, which makes that complexity really frustrating. Complexity where it is necessary is fine of course, but I feel like IDEs like Eclipse frequently cross that line.
Eclipse isn't really just an IDE, it's almost an OS in its scope and complexity. I agree it's way too complicated and bloated, and worse: the rewards aren't there like they are in IntelliJ or Visual Studio. It's not as polished and not as easy to set up.
The whole point of integrated software is to be able to do everything in one place: debug interactive in editor, hover to inspect values, edit the code and have the ide recompile and reload the changed program.
The IDE that does that likely does other things, but having unused features should neither add complexity nor incur performance costs!
The complexity of setting up a dev env like the above in emacs or vim is absolutely staggering, but the complexity lies in the configuration and setup, not in the finished env, that's the difference. Personally my tolerance for "configuring" my tools (meaning gluing, telling my editor and profiler where my debugger is) is nearly zero.
The type of complexity that bothers me is complexity that I have to wade through on a day-to-day basis when trying to find features that I do want to use. For example, lots of menus, each with tons of menu entries and submenus. That sort of complexity won't (or at least shouldn't...) impact performance in a meaningful way, but it exacts a psychological toll on me.
As you say, Vim's complexity is different. It has a lot of nonsense built in, but unless you go looking for it you will probably not become aware of it. It also can be complex to set up correctly like you mention, but that sort of complexity bothers me much less. That's more of a "once and done" thing, which I find much more tolerable.
My tolerance for configuring software is actually very nearly pegged at zero, unless I believe that the configuration I am creating will still be useful for at least 10 years. This rules out configuring just about anything for me, with very few exceptions. My current significant configurations are for vim (the emergence of neovim gives me some concern) and zsh (aliases and basic keybindings only. I am confident that any shell I'll be using in the future will allow aliases). My window manager, browsers, etc all use their default configurations (I got burned by Awesome WM changing their configuration scheme a few years ago).
Configuring Eclipse to be tolerable is probably possible, but I have zero confidence that any such configuration could last even two or three years.
