I just upgraded my Windows 10 Home 64-bit installation to the latest
Windows Insider preview version:

    Version: 1607
    OS Build:  15007.1000

I'm now getting this warning from Cygwin, but only in a CMD window
(not XWin).  It doesn't seem to affect anything negatively.
cygcheck.out attached (I got the FAST_CWD warning three times during
the generation of the file):

C:\Users\jjrei>uname -a

Cygwin WARNING:
  Couldn't compute FAST_CWD pointer.  This typically occurs if you're using
  an older Cygwin version on a newer Windows.  Please update to the latest
  available Cygwin version from https://cygwin.com/.  If the problem persists,
  please see https://cygwin.com/problems.html

CYGWIN_NT-10.0 JJR 2.6.1(0.305/5/3) 2016-12-14 14:33 x86_64 Cygwin

-- 
Jim Reisert AD1C, <jjreisert@alum.mit.edu>, http://www.ad1c.us