I don't have first hand experience with deja-dup, but according to Google it uses duplicity which in turn uses tar for its backup archives. It is the nature of tar that it does not support direct access to files but instead has to scan the archive until it comes across the desired file. There are alternative archive formats such as dar which provide direct file access. So if fast access to single files is a requirement for you, another backup solution might do a better job for you there.