Not quite correct as the function that speeds up scans is the Persistent cache and this doesn't cover all files.
Use persistent caching - if persistent caching is used, the information about the scanned file is stored in the permanent memory. This means it is not lost after a system restart and it is also not affected by virus definition updates. Consequently, persistent caching is suitable only for files which are guaranteed not to contain any virus infection e.g. operating system files, files signed by trusted publishers, or other files covered by the avast! whitelist. This box is checked by default; if you want all files to be scanned regardless of their trust status, this box should be unchecked.