Deleting large files is extremely slow

This is  not fixed in Windows 10 preview,  Version 10.0.10074 Build 10074.  Long time to calculate 20GB of files and for the first few minutes after it began deletion  it estimated  it would take over 3 hours to delete them.  This is under a 2.8GHZ dual core, with almost 2GB free ram, deleting files from a SATA drive, and with Windows Explorer  the only thing using  more than 3% of the CPU cycles.  

After about 4 min. that estimation changed to 45 minutes and remained there for about 5 min., then changed to 30 min.  8 minutes later that estimation changed to 24 min.  11 minutes more that changed to 21 min. Then it increased to 22 minutes about 3 min later.  About 4 minutes later that changed to 5 seconds, where it remained for about 10 minutes, then changed to a finally accurate 4 minutes.

Thus, despite the time taken for calculation,  its estimation was fairly useless. while the time taken to delete 20GB of files

I could have right clicked and chosen Unlocker (thank God for freeware, but opt out of any bundled additions) to delete at next boot, but  did not. 

Copying files also  requires calculation of time, resulting in various conflicting estimations of time remaining (7 min. to 23 min.  to "more than one day," etc. for 20GB, ),  

 But I appreciate the many things  Windows does do so well