If I have a clip on the timeline with fx and subtitles rendered, and then I say extend the clip by 1 frame, the whole clip needs to be rerendered. Why? If I blade off a piece at the end, same thing. In FCP 6, only that frame or piece would need re rendering...
Any solutions?

Anyone?
Even if I extend or shorten a gap clip in the timeline, with no other clips attached above, suddenly many other clips need to be rendered. Work is slowing to a crawl....

I know NOTHING about FCPX but this reminds me of the bad old days of editing long GOP codecs.

Are you editing with AVCHD or some other long GOP codec? If you transcode to an intraframe codec like ProRes, does the problem go away?

This sounds wrong. Changing a gap never unrenders anything unless you are building over that gap with connected clips. A couple of questions:

Are you editing in the same format as your clips?
And as Shaun asked, are working with AVCHD or MPEG2HD files?

All footage transcoded to pro res for editing, and the sequence matches the clips.
The sequence is pretty complex with lots of layers of clips, color corrections, MB looks and neat video noise reduction, but clips should stay rendered. When re-opening fcpx, it also sometimes loses random renders....
Early on I made part of the sequence a compound clip, but I never collapse it...
I made a new sequence (with the appropriate settings) and pasted the whole timeline into it. Same problem.
I wonder if the compound clip is making things funky?

Neat has you beat here. It's a particularly complex correction filter, I use it rarely and always turn it off until the final render. The same with the "makeup" filters. You should find that some filters will cause this problem and others don't. Also if you key frame the parameters in a filter it will un-render the whole clip with any change in the clip or over the clip. I sometimes split the clips so that the keyframes are outside the area I am changing.

I will look into the neat beating I am apparently taking...
Thanks for the input!