Are you on an Active Directory domain, but not immediately connected to it?
While I don’t have the “answer” as to why MinGW would be slow, it has been my experience that computers which belong to an AD domain, but are unable to reach the AD controller, have a delay in starting executables (such as rxvt.exe) and currently running ones experience a pause or stutter (such as emacs, which is built using MinGW).
I am still investigating to determine the actual cause of this behavior, but thought I would mention it in case it applies to you.