If Lightroom becomes sluggish during Spot Editing; the cursor does not move smoothly, it becomes hard to draw irregular shapes to cover blemishes, etc. This happens when making a lot of corrections with Spot Edit. The way to recover from the problem is:
1. find a clear stopping point, like end of a Spot Editing pass across the magnified image
2. export the image,
2a. naming it clearly so it will not be confused with the source version of the image
2b. ensuring that no unwanted copyright or captions are accidentally included in the exported image
3. import the just exported image back into Lightroom
4. contine spot editing.
Repeat steps 1-4 above as often as necessary.
When Spot Editing is complete, delete the intermediate images on which the Spot Editing is incomplete.
This works because Lightroom exports the image as last modified during the Develop function. When it is imported, all of the changes done are simply part of the imported image. In effect, Lightroom gives a clean sheet of paper for continued work. There is a caveat... it is not possible to undo changes on the imported image that were made to the source for the imported image. This is a small price to pay to recover Lightroom's performance when making lots of changes with Spot Edit.
As soon as you export the RAW file and reimport it is no longer RAW. If you are going to do this you might as well just send the image to Photoshop and do all your spot healing there. Its a one time thing and the tools in Photoshop will not suffer the slowdown. I have never had this extreme slowdown for spot editing in LrC but you may be doing a LOT more of it than me.
Try turning Off 'Use Graphics Processor' in Edit> Preferences> Performance. If you have a fast CPU the Spot Removal tool should be more responsive. If you're doing a lot of Spot Removal on one image then Edit in PS Spot Healing Brush works better than LrC.