Why Does It Take So Long to 'Wipe MFT Free Space'?

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I have cCleaner (free link in the picture-description). I run it a few times a day--mostly whenever I have more-than-5-MB of junk-files on my disk. In its advanced-options, there's the option to "Wipe 'Free Space'" (a.k.a. (either in previous versions or elsewhere in the 'wipe'-process) 'MFT Free Space'). That options usually NOT chosen, because the process takes 5-or-10-times LONGER to run. The few times I check it and run the cleaner, I come back and find the 'time left'-indicator still counting UP (as if the program is still FINDING Free Space to clean). So I finally 'asked Microsoft Bing' "what MFT Free Space is," and I found out ... https://www.bleepingcomputer.com/forums/t/557676/ccleaner-wipe-mft-free-space/ UncleXNL's explanation there starts out "... The computer NEVER deletes files, instead it sets the file's deleted bit to TRUE. Now the file is still on disk, but the free space could be overwritten. Until that happens any undelete tool can recover the file in a second ..." Elsewhere, users explain that MFT stands for Master File Table ... I still don't understand WHAT THE MFT-WIPE IS DOING THAT TAKES SO LONG!!!