Try comparing the boot times of a Windows system that doesn't have MS Office and a similar system with MS Office. With current hardware and versions of MS Office, I usually see about five seconds of extra lag during boot. If you have the OpenOffice quickstarter loading, that extra five seconds is visible to the user as it loads after the user has signed in. If you don't use the quickstarter, then you see it when you first load OpenOffice manually, as you say.
KOffice, in my experience, loads significantly faster than either MS Office or OpenOffice, and doesn't impact boot time. On the other hand, its support for ODF and the way it treats text styles are practically the definition of suck. I have very high hopes for 2.0, though, and will certainly give it a shot when it's available.