Check that you have enough free space for iMovie to function efficiently. (Apple Logo/About this Mac/Storage.) Make sure that you don't have any CPU intensive apps running at the same time as iMovie. (Applications/Utilities/Activity Monitor).
If all O.K, try opening iMovie while holding down the Option and Command keys and selecting 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. Now see if the editing lag is improved.
If still no luck, you could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Another thing to try is to reset your Systems Management Controller. It is easy to do. See this link for instructions:
https://support.apple.com/en-us/HT201295
All else failing, try a redownload of iMovie 10. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
-- Rich
