Again I've stayed with SyncToy v1.4, as I found v2.1 to be also too slow to be of any use.
First I ran v1.4 to make sure everything was already synched and echoed, and then I installed v2.1 over v1.4 (which kept its settings and upgraded it).
Then I previewed two known-to-be-identical folders, each 7.16Gb (containing 53,817 Files and 2,594 Folders). SyncToy v2.1 took 11 minutes and said that it would need to overwrite 46,650 files (and that 60,972 files did not require action). This was too slow and was incorrect about having to overwrite any files.
So I uninstalled v2.1 and reinstalled v1.4, and did the same test. This time it took 2 minutes to compare the same folder pair and no files needed copying (except for a few SyncToy_*.dat files left by v2.1).