MacOS – Is the time required for incremental Time Machine backups greatly affected by the number of files

Since you don't want no as an answer, and since Time Machine is proprietary software that you cannot modify you only have two options (don't complain, you asked for it)

• convince Apple to implement the missing feature
• implement your own version of Time Machine

You could modify backups from the terminal or with the Finder: backups are just directories with hard links but beware that you will most likely make them unusable by Time Machine (as Time Machine relays on the fact that the backups are as they were created).

There is a function in Time Machine "Delete all backups of" which removes backed up copies of a file. You could research if this is scriptable (Apple Script or terminal). I had no luck but seems the only way to periodically clean up a backup (with cron or similar)

You didn't specify which virtualization software you are using. If it's VirtualBox there is a workaround to reduce the size of the backed up data using snapshots: http://blog.matteocorti.ch/?p=331

Be careful with sudo and making sure you pick the correct Mac's files since there is no undo or confirmation of the following command:

sudo tmutil delete /Volumes/drive_name/Backups.backupdb/old_mac_name

The sudo command needs your password (and it won't echo to the screen, so just type it and pause to be sure you're dating the correct files before pressing enter). If you want to be safer, you can pick one snapshot to delete first to be sure the command works as intended. This is nice since it could take hours to clean up some larger backup sets and you want to leave the Mac confident it's deleting the correct information store.

You can use the tmutil tool to delete backups one by one.

sudo tmutil delete /Volumes/drive_name/Backups.backupdb/mac_name/YYYY-MM-DD-hhmmss

Since tmutil was introduced with Lion, this will not work on earlier OS versions.

If you want to get the current directory of backups (there can be multiple destinations defined and only one will be "current")

sudo tmutil machinedirectory