Using the Time method to scan for duplicate files is the fastest (as the file's data isn't scanned) because the comparison is done using the file's date and time with an optional time threshold.
You may use this option on synchronized files which you've already scanned at least once before. Only the file's modified date/time stamp will be used.
File time threshold
This option is only used if comparing files by time and the default value is 2 seconds. In Windows 95/FAT based systems, file time stamps have an error gap of 2 seconds or less. So you may want to adjust the threshold according to the error margin of your source and destination files.
Say, from an XP (with NTFS file system) computer to a 95 (with FAT file system) machine, set the file time threshold to 2, but from XP (with NTFS file system) to UNIX based operating system, set it to 0 (zero). Check your OS documentation about file time accuracy. Some information mentioned than Windows 98 and better have no need for a file time threshold other than 0 (zero), but certain tests on our labs shows than Windows 98 may have this timestamp problem too.