Anything in the dock is just an alias file and of no consequence regarding computer speed. They probably reference the same library. Even if they didn't, I don't see any issue. iMovie will open in the previously opened library.
I would try a few more diagnostics:
Try booting up in Safe Mode and, after everything loads, immediately shut down and reboot in normal
mode. Often that clears out conflicts Then see if iMovie works properly. Here's a link that explains the process for you type of computer.
Start up your Mac in safe mode - Apple Support
Another thing you can try is to open iMovie in a new library and see if performance is better in the new library. Hold down the Option key while opening and choose a different library or create a new one.
Redownloading iMovie sometimes cures issues.. 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 and return it to the Applications folder. However, it should work fine and give you the highest version of iMovie that your OS will run. Now see if the performance is any faster.
Two other things that you can try is to reset your Mac's System Management Controller and PRAM. They are easy to do.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
How to reset the SMC of your Mac - Apple Support
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM:
Reset NVRAM or PRAM on your Mac - Apple Support
-- Rich
