Vista Defrag with Perfectdisk

I have noticed here when defragging Vista with Perfectdisk that if I stop and disable Superfetch in services then run PD I seem to get a better defrag.

After PD finishes I reset Superfetch to auto and restart.

 

I am uncertain of what "superfetch" does. Is there any downside to enabling/disabling ?

I noticed that if I choose a "boot-time" defrag of system files on a drive with some restore points, the restore point files don't get defragged. But If I try an ONLINE defrag of system files, I will get a message asking me if I want to "Force all open Handles Closed", to which I say yes. Then, it says it needs to reboot to complete that operation, reboots and defrags the previously undefraggable restore point and other system files.
Maybe "Forcing all handles closed" is equivalent to disabling Superfetch ?

I am just shooting in the dark here.

 

Vista boots into desktop quite quick here then after about 15 secs there is heavy hard drive activity for around 30 - 40 secs.

That seems to be Superfetch kicking in and loading most often used apps into memory.

As soon as Desktop comes up you can open services and you will see Superfetch hasn't started as yet even though set to auto.

Or you can bring up Taskmanager asap and you will see the highest memory using svchost.exe process kick in which relates to Superfetch.

Or at least that how it seems to work here and I have tried to get Superfetch to start as soon as desktop comes up but ain't been able to do so as yet.

Superfetch does seem to make Vista snappier except for the lag when it first starts working.