Is there a way to stop the optimization process behind "Optimizing Your Mac"?
What you describe is far worse than the normal optimization phase.

If your Mac is so sluggish that the cursor movement and launching of apps is unworkable (delays more than 60 seconds per action) and persists past a reboot, you'll need to get the hardware checked as the storage system is likely failing due to software or hardware failure.

To troubleshoot this:


back up all files you care to ever see again
disconnect the backup drive (and test restore to another Mac if that costs less in time and $ than you'd spend to recreate the files or compensate you for the loss - i.e. wedding and/or baby pictures, etc...)
Reinstall a clean OS - import no data, set up no network, no Apple ID, no iCloud. Run that system until it's idle in activity monitor and there's no delay launching apps from spotlight.
Test opening any three apps you care from Launchpad


If this OS is slow, troubleshoot the hardware further.

If this OS is fast:


connect to the internet
update all the software
install any critical software you need to work/test (this rules out a software conflict with the software you were running when the machine was stuck)
open Activity Monitor and be sure the system is idle and performs properly
now decide to import your user files, connect to iCloud and Dropbox and any other tools that can cause slowness and external dependancies. If you don't want to set things up by hand, you can wipe this test OS and reinstall and choose to restore your backup in one shot.


Good luck - troubleshooting hardware/software isn't a hard concept, but being organized, limiting change, and testing after each step is what makes it sometimes tedious and worth outsourcing.