iMac has started to operate really slow when editing in FCPX

Editing is taking so long,,, even when I scrub over video clips my system stops & starts and lags.
It used to be completely fine and operate really quick.
My Main HD is only half full, I have 16GB of memory and my video files I am editing with are stored on a External drive.
Please HELP??????

How full is your external drive? How is it connected to your iMac (USB 2, USB 3, Thunderbolt, via any hubs)? Is the external drive an SSD or HDD?

You could test your external drive using this (and your internal drive) and report what you find:

apps.apple.com/us/app/atto-disk-benchmark/id1479103658?mt=12

If you external drive isn't fast enough, you'd experience what you describe. If the drive is more than around 80% full, that also could cause issues.

This is a nice first step to take in troubleshooting things. If your drives checkout, you can try other things...