Comment 2 for bug 579804

Have you considered a background task to update the tags?
Tagging represents a lot of work by the user and this can be easily lost. (e.g. a user backing up their files to CD and reinstalling the OS without realising that the Shotwell database has not been backed up).
If the performance is too critical I'd recommending having the option like F-Spot - we would deliver it enabled and more advanced users may consider disabling it as a speed up.