iMovie takes an hour or more to load
when using iMovie in my Mac, it is excruciatingly slow. It takes over an hour to load when it is validating library.
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Sometimes very large libraries can take longer to load. You might try creating a new library and see if it loads faster.
Never hurts to try deleting preferences, as that will reset iMovie's defaults.
Deleting Preferences
Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears.
iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Also, you can try the following two procedures, that are very easy to do -- rebooting while holding down certain keys:
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM.
https://support.apple.com/en-us/HT204063
-- Rich