Given registry access is done in memory and whole hives are not necessarily automatically loaded anymore, MagicAndre is correct - unless the registry hive is gigantic (1+GB, and even that doesn't take much on a newish system) and horrendously fragmented on disk ... and it's not an SSD), loading the portions needed into memory don't take very long at all during boot (perhaps a second or three). Also, given the hive as a whole is no longer loaded in it's entirety, "cleaning" it or compacting to clear white space is only going to give you back some MB on disk - it isn't likely to improve performance in a perceptible way at all unless the system is *really* old with a *really* slow 5400RPM or slower disk as the boot volume. In my experience, only then would "cleaning" the registry provide performance gains. Having duplicate entries in places could cause *application issues* if the app itself doesn't like such problems, but those are application stability issues, and it won't make Windows (or the apps running on it) go any faster in any perceptible manner. As MagicAndre said, this has been this way since XP in 2001, and even Windows 2000 did a better job of 9x loading hives into memory so it wasn't really an issue there either. This is one of those nuggets of "legacy knowledge" that keep getting trotted out as a help when, in fact, it's at the least fairly useless, and potentially dangerous to system stability (I've seen registry "cleaners" totally break boxes), with no real upside that would be gained by doing so.