I was having serious lag problems making Yosemite unusable. Was about to revert to Mavericks when I discovered uninstalling Little Snitch resolve the problem. There is a nightly build that works with Yosemite apparently but I haven't tried it.
There is an iMovie workaround:
- Find iMovie in Applications
- Right-click and select "Show Package Contents"
- Open "Contents" folder
- Open "MacOS" folder
- Double-click iMovie
Apparently works for some other apps as well.