Atom doesn't use 80MB, it uses 800MB. On the other side, my Eclipse installation uses ~1100MB and runs whole array of analyzers and background stuff that an IDE should run. Atom runs nothing at the back or provides the same functionality while using 800MB. Also, Eclipse is portable around all major operating systems.

Resource usage can be justified if the that resources is used toward something meaningful. C is "slower" than assembly, yes; however both assembly can be inlined into C, and magnitudes more can be done with C at near-assembly speeds.

I develop in C/C++/Python mainly and in some other languages for fun and curiosity. Python is easy, but it's not as performant unless the underlying library is native, hence nobody is using it if it can't obtain very high performance in my field (scientific computing, HPC, and related areas).

Actually, writing a low performance GUI is pretty hard since most of its cycles are idle. If you can write an application with a GUI which slow to respond to low-complexity, common tasks (naive text editing, input, menus, moving windows), something is very, very wrong (to be clear, GUI can trigger resource intensive background tasks, but that's not what I'm saying).