That's a very odd result, especially given that the patch was for the actual code (aswMon2.sys), not the definitions - would be strange that definitions would be able to cause bad behavior in code. Not impossible, mind you, but very strange.
To test this, I installed the 110723-0 definitions on my system as well - again, WinXP SP3 with Retrospect Express 7.6.3.102 and Avast! Free 6.0.1203. I then cranked up Process Monitor and ran Retrospect, but saw no slowdown nor any of the activity associated with the slowdown that I reported before, so unfortunately I'm not able to reproduce this behavior. Has anything else changed with respect to your system, or did it only start doing this on reboot or something? Have not rebooted my system yet, to be fair... The only thing we might've done differently is that I did not overwrite the old aswMon2.sys in Avast!'s INF folder - rather, I renamed it aswMon2.sys.bak - don't see why that should make a difference, though...
DS