When syncing 2 disks, the initial compare of files apparently does every file whether the directory tree they are in exists on the target drive/directory or not.
Take worse case: when syncing a drive to a new formatted but EMPTY drive, DO does a compare of each file on the Source drive. On a very fast machine, with a 200gb of files, this took just over 24 hours... if the 2TB drive had been full, that would have been over a week.... OUCH! In this case, doing the compare by looking at the target on a directory tree basis would have marked all the source files in the time it takes to just mark them and then they copy could begin. The same situation occurs when trying to Sync two disks that have had new directories added to the source drive. Once a parent source directory is found missing on the target, all the files under that source directory can be marked to be copied. In addition, if there is a way to determine that directory trees on a source and target disk have not changed (might be good to be an option if this is risky), this would save comparing all the files under the top level of the directory structure that is unchanged.
That doesn't sound like what should happen, at least.
Sync does not compare file contents, only names, dates and sizes.
When you say it took 24 hours, do you mean the initial comparison where it's working out what to copy, or the sync itself where it's actually copying the data?
Is that true even if byte comparison is selected?
If One-way Copy / Byte comparison is turned on then sync will compare the contents of files, but only if their paths, names and sizes are all identical. So that wouldn't happen in the scenario where the destination folder is empty.
I just did a test on a 7TB folder, syncing to an empty folder, and the Comparison Results dialog appears almost instantly when I click Compare. So it isn't reading the file contents to do the comparison. (Obviously copying the data will require reading and writing all the file contents, but that happens in the second stage.)