Just suffered from extreme slowness. My Emacs had been running for about 1 hour and I was having two org-mode buffers, one file has 3800 lines, 168 KB bytes. And the other has 76,600 lines, 4,267,327 KB bytes. Both files had been opened for awhile. Didn't have problem in the beginning, but then typing in the bigger buffer became extremely slow, took at least one to two seconds for a typed character to be displayed on the screen. Killed the buffer and then reopen it didn't help at all. At the same time typing in the other smaller buffer didn't have the same slowness issue. Restarted Emacs and the issue disappeared.
One of the problem I have noticed for a long time is that org-mode could become slower and slower with use, which was exactly what I had been experiencing just now.