Hard to say; the File resource is the only one written in native code, so we can't really see what's going on there.
I can tell you that the DSC cache folder is also automatically cleared every time a new version of the configuration is applied with Start-DscConfiguration, or when a new version is downloaded from the pull server. So the cache only really helps when the system is being checked for drift from an existing configuration.
Thanks for the reply. I've removed this from my configuration for now.
I really need this to work, so I'm going to try write up a custom resource that simply uses robocopy mirroring if a checksum file differs in its content.