IntelliJ IDEA is my favorite IDE for all Java development related work. I do moonlight with Eclipse as well from time-to-time, but it is very less and also the sheer speed and power of IDEA pulls me back every time.
I have found the following tuning parameters to be effective for my day-to-day work for fast loading, caching, etc and for the IDE to function smoothly:
If you are on 64-bit Windows OS, please add the above config in idea64.exe.vmoptions file and if 32-bit, please add these tuning parms to idea.exe.vmoptions present in IDEA_LOCATION/bin folder.
Also just in case if you are not aware, on a 64-bit machine, if you want to launch IntelliJ IDEA in 64-bit mode, you need to launch idea64.exe and not idea.exe. For this, ensure you have
IDEA_JDK_64 environment variables pointing to your 64-bit Java install folder.
Finally, the above IntelliJ IDEA tuning parameters hold good for IntelliJ IDEA installed on a Linux machine as well.