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Indeed, the main problem with Vista's superfetch is that even with low priority for its CPU and i/o thread, it can cause major slowdown during boot because of superfetch consuming the hard drive when you just want to launch an app immediately.
In Vista, superfetch starts before you log into your system. In a multi-user setup, it can't guess very well which applications to preload first.
For a single user, it actually does work well (at least for me, and depending on how much RAM you have for superfetch to preload).
In Windows 7, microsoft is further lowering the priority of the superfetch thread, and also delaying it from starting until after the desktop has been loaded.
Windows 7 also in general just has less processes and services at startup, with a rewritten service stack with better delayed loading (partial loading on demand).
I've found the responsiveness at boot (in Vista and XP) is tied directly to the random throughput of your hard drive. When I upgraded to a 10K rpm Raptor, I can now boot windows and pretty much immediately open any application. Superfetch doesn't really get in the way of the Velociraptor with its low seek time. A typical 7200rpm is at least twice as slow, and a 5400 is even slower. Most laptops of course using the slowest option, sometimes even a 4200rpm, or a poor SSD.
But for more generic hard drives of the 7200rpm variety, I have seen Superfetch make the machine unbearable to use during boot. Windows itself would freeze and pause waiting for hard drive i/o.
Some of it I attribute to bad drivers, but even with a powerful modern machine (q6600,4GB,500GB 7200rpm) I can notice the pauses for i/o.
Similarly, its the same situation with poor SSD's that can barely read or write random i/o. It's all about latency.
Overall though, if you can wait for Vista to fill the superfetch catch (50MBps to fill 4GB?), then your experience after that would be better assuming Vista prefetched the correct programs. Edited 2009-05-12 07:20 UTC