You type in a letter and that starts off a cascade of computations, incremental compilation, table lookups, and such to support syntax highlighting, completion, etc. and then it updates whatever parts of a dynamic UI (the user decides which widgets are on the screen and where) need to be updated.

It almost has to be done in a "managed language" whether that is Emacs Lisp, Java, etc. and is likely to have an extension facility that might let the user add updating operations that could use unbounded time and space. (I am wary to add any plug-ins to Eclipse)

I usually use a powerful Windows laptop and notice that IDE responsiveness is very much affected by the power state: if I turn down the power use because it is getting too warm for my lap, the keypress lag increases greatly.