The rich text editor relies on javascript to do it's job. As it's a WYSIWYG kind of thing that means it has to always be "ON" in case you start typing something. Since the reply box at the bottom is always open, then javascript is always getting processed by the browser. Which means the CPU is occupied, which means everything slows down.
Not an issue on modern Macs, big issue on our older Macs.
Honestly, BB code isn't that hard so for me it wasn't a big deal to turn it off.