I inadvertently pasted several megabytes of characters into the find field of the search and replace dialogue box. The performance of TM dropped significantly whenever the dialogue was displayed as a result.
I tried using the CLI tool defaults to remove the find history from the preference file and even reverted to an earlier version of the preference file (courtesy of Time Machine) to reduce the preference file size from 4.5M to 25K. Unfortunately starting TM again somehow restored the preference file so I was still left with the problem.
It turned out the fix was simple - just select a few characters and Command E to make them the new find string. Repeat with a different set of characters a dozen or so times. This pushes the offending search string down the history list until it is discarded.
Dave.