The extreme slowness happened again just know. When this happened,
`org-end-of-line' command took forever until "C-g". M-x
org-element-cache-reset
worked this time, i.e., after running `org-element-cache-reset' command
`org-end-of-line' became fast again.

I was using commit "ca6ecf9", and the buffer was 38xx lines 168K in size.