I had exactly the same problem after upgrading from 2.1.3.
The spell checker was working beautifully before the upgrade. It was fast and efficient. After the upgrade everything slowed down terribly–painfully, with “please wait” messages or else the Ajax error. So much for an “upgrade”!
I solved the issue by simply renaming the spellchecker directory in wp-includes/js/tinymce/plugins and copying in my old spellchecker directory from my previous version (I’d saved everything before the upgrade). I then logged in again and the spellchecker worked as it did before.
I haven’t fully tested it but it seems to me like the spell checking is back to normal. Something else might be affected by doing this but I haven’t noticed any problem yet.
