Does rdiff-backup always compare checksums of files when restoring? Since computing checksums for large files can be costly, is there a way to check the modified file on the basis of last modified timestamp, as like rsync?

During a restore, rdiff-backup always computes the checksum of the data it pulls from the mirror or reconstructs from the increments and verifies that against the checksum that was stored for that version of the file**. I'm not quite sure what you are asking. rsync has the ability to compare the checksums of the source and destination in order to determine whether the destination needs to be updated. That doesn't make sense for rdiff-backup during a restore operation, since it either (a) refuses to overwrite an existing file, or (b) with the "--force" option unconditionally overwrites existing files.

** Very old versions of rdiff-backup did not store checksums. Missing checksums result in a warning during restore.