Be default, Windows 7 disables Superfetch, ReadyBoost, as well as boot and application launch prefetching on Solid State Drives (SSD) with good random read, random write and flush performance. These technologies were all designed to improve performance on traditional hard disk drives (HDD), where random read performance could easily be a major bottleneck. Because of the design changes, Windows 7 powered PCs with SSDs run fast.

To understand how these design changes improve overall system performance, read this team blog post by the Windows 7 product group.

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