Probably because Atom uses GPU-accelerated rendering where IntelliJ does all Java (with some C++ bindings) rendering, thus all on CPU. Intellij’s ‘rendering’ performance problems would likely be solved if they used a decent GUI toolkit (like QT5) instead of the horror amalgamation it uses currently (a weird mix of C++, Java, and some really low level bindings last I looked, all to avoid Swing).
If so you should submit a bug report, they have been really fierce on performance bugs the past year and things like it dying become priority 1 bugs. Are you sure you do not have a bad plugin installed? It has some great debugging tools built in.
For note, KDevelop uses QT. ^.^
