vc-dir operation is very slow on large git repositories in Emacs 26.1

[Alex Harsanyi <alexharsanyi@gmail.com>]

I have a somewhat large git repository at work (51556 files) and
Emacs 26.1 is very slow when updating the *vc-dir* buffer: I
start emacs with the "-Q -no-site-file" and it takes
approximately 2 minutes to update the vc-dir buffer. Emacs 25.2,
on the same machine and repository, will complete this operation
in a few seconds.  This is on a Windows machine and I use the
prebuilt binaries downloaded from the gnu.org website for both
versions.

The operation that takes a long time is the ls-files
operation, "git --no-pager ls-files -z -c -s --", but running
that command in a terminal completes in less than a second.

The rest of Emacs is fully responsive and I can continue editing
while the update is running in the *vc-dir* buffer.  Also other
version control operations (diff, check-in) are responsive.

I cloned the Emacs git repository (which has 3683 files) and a
vc-dir operation in Emacs 26.1 takes 7 seconds on my machine,
compared to less than 1 second in Emacs 25.2.  I noticed this
speed difference on my home computer as well.

I was wondering if others have noticed this issue?

If not, I would appreciate some suggestions on how to further
diagnose the cause of this issue, as it seems it is related to my
computer.

Thanks,
Alex.