[tomsem]

Here's a theory: your 'My Clippings.txt' is getting really big, and it takes Kindle longer and longer to find the end of the file each time you go to add a new note. Every note probably requires Kindle to open the file, seek to find the end, add note, close the file so that change is saved.

I would back up this file to your computer and then delete it from Kindle to see if performance improves (Kindle will happily create a new, empty one).

Note that if you rely heavily on notes in general, you should be backing up this file regularly anyway, because only notes for Amazon books are saved with your account, and My Clippings is not. My clippings is really the only easy way to get at notes for non Amazon books, e.g. if you want to copy/paste to some document you are creating.