I know this was posted a while back, but the following applies to this thread as well as many other "change this in your registry to speed your computer up x1000000!!!!" style threads.
Could the OP (or anyone else advocating that this dramatically decreases boot time) explain exactly what the above registry key controls, how decreasing it speeds up boot time and the possible side effects of doing so? If not then why suggest it in the first place? If I said "here, have a random exe file, double click it and let it run and it'll make your computer start 5x faster" would you? Would you run a virus scan through it or at least check it out first?
The same should apply to registry tweaks - they're flying blindly around all the time but in reality I've found very few that make any noticeable difference, and those that do tend to be fully documented in terms of what they change and why that makes such a difference (pretty much always with a side effect, even if that's irrelevant to the specific user.) Yes, we all bash MS (me included) but they're not completely stupid. They wouldn't set all their registry values higher for the heck of it if putting them down by a few thousand increased boot time with no side effects whatsoever.
Since no-one's done it already, let's apply some logic to this and head over to the official documentation to see what this key actually does:
The StartupDelay entry controls the time delay after a system startup before Indexing Service, an automatic system service, starts any scanning or indexing work.
...and look, we suddenly see why a nice long delay is sensible - do you really want your indexing service thrashing away the moment windows starts up? I certainly don't. In fact from the above I'd say decreasing it is likely to increase the time to get a usable desktop, since you're firing off the disk-thrashing indexing service the moment windows kicks into life (or very shortly afterwards.)
Funnily enough, looking at the documentation also shows why the key doesn't exist on Vista / 7:
Note Indexing Service is obsolete as of Windows XP. Instead, use Windows Search.
So, this is directly related to the indexing service. This was replaced with windows search which is the default in Vista onwards, and has also been available as an add-on to XP for ages.
To sum up? After doing the research, this hack will do absolutely nothing if you're running windows search with windows XP or any newer MS OS. If you're running the old indexing service with windows XP, it's probably going to increase your startup time to a usable desktop, not decrease it.
Hands up who's rushing to apply this tweak now?