I downloaded the HEIC extension and the instructions say to clean media cache, but I don't know how

@amyvegte, now that the Organizer recognizes your .heic files, it is probably passing them through media analysis (face recognition, smart tagging and auto-creations).  This analysis can take a long time, depending on your computer specs and the size of your catalog, and it can slow down the performance of Elements.  To confirm if this is the case, go to Edit>Preferences>Media Analysis and uncheck all of the boxes.  Does this make a difference to the performance (after a reboot)?  If yes,  and you want to use the media analysis, you should check the relevant boxes and leave your computer running overnight (or over several/many nights depending on the size of your catalog).

If Media Analysis is not making any difference in your slowdown, what are your computer specs, how many photo and video files do you have in your catalog, and where exactly are you seeing the slowdown in Elements?

ADDED: One other thing for you to try is to Optimize the catalog, using the File>Manage Catalogs dialog.  This may help with drawing the .heic file thumbnails, although it can also take a while to complete.

I don't have an AppData file under c:\users\[myname].

Delete all of the files and folders that are contained within the Common folder which may be found at:

C:\Users\[Your Name]\AppData\Roaming\Adobe\Common\