The appListCache folder is used for more than just Unity apps - the contents of the Start Menu are stored there as well, to be used when the VM is not running (suspended or powered off). It also acts as a cache that reduces the amount data that Fusion needs to read from Windows when the Start Menu is updated, which avoids a lot of duplicate disk IO and speeds up both startup and resume performance of VMs.
The entire appListCache should be very small (in my experience it has never been larger than 5 MB). The majority of the contents do not change very often (only when the guest apps or menus change), so I highly doubt that it has any measurable impact on backup speed. The virtual disks that are gigabytes in size will change much more often, and will slow down backups a lot more.
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If you really want to prevent the contents of the appListCache folder from being created (at the expense of limited Start Menu functionality and more disk IO at startup), you can either make it into an empty folder with non-write permissions, or create a blank file with that name, which will prevent it from being used as a directory.
