The real time playback of un-rendered clips is handled so well that rendering everything all the time is just a waste of processing power and disk space - remember all render files are ProRes.
Turn it off: Final Cut Pro / Preferences / Playback
Then you can render everything by pressing SHIFT + CONTROL + R or selectively render by selecting clips and pressing CONTROL + R.
Am I just trying to make it more like FCP7? Er....maybe.