IntelliJ IDEA is my favorite IDE for my day-to-day work.
I do moonlight with Eclipse as well from time-to-time, but the sheer speed and power of IntelliJ pulls me back to IDEA every time.
I have found the following tuning parameters to be very useful for fast loading, caching, etc for IDE to function smoothly:
If you are using Windows 64-bit, please add the above config in idea64.exe.vmoptions file and if its 32-bit, please add these tuning parms to idea.exe.vmoptions present in <<IDEA_INSTALLED_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 and IDEA_JDK_64 environment variables pointing to your 64 bit Java Installation folder.
Finally, the above IDEA Tuning parameters can also be used for an IntelliJ IDEA installed on a Linux machine as well.
Update on 05th April, 2015: After a fair bit of time here, I have moved on to GitHub hosted Octopress blogs. Please find me on http://P7h.org henceforth for all new updates.