Have you checked
http://www.cerious.com/performance.html ?
Are you using the same type and size of images as before?
Is editing images in ThumbsPlus the only thing that is slow, or do other tasks in ThumbsPlus, or other programs, now work more slowly too?
You could try this: Exit ThumbsPlus, locate your Thumbs8.ini configuration file and make a backup copy of it. Then open it in Notepad and add this section to it:
[Virtualmemory]
Minalloc=0
This would disable the built-in virtual memory manager - but that will likely only make a difference if not just the editing but also the loading of image files has become slow.
(ps: This is the correct place for your question, so I have removed the cross-post from the General Discussion section)