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RE: X11 forwarding extremely slow (unusable) after Cygwin upgrade on Dec 23rd

The problem is with cygwin 3.1.0-1, released on Dec 16th, or thereabouts.

I decided to track it down by re-installing the earlier working release from
the Cygwin time machine at, and then
incrementally upgrading it until the problem showed up. It turns out that it
was introduced in the Dec 16th release, which made just one change -
upgraded cygwin from 3.0.7-1 to 3.1.0-1. With the Dec 15th release all works
fine - nice fast response in the X11 windows, with the Dec 16th release the
same programs in X11 windows are really, really slow.

I can roll cygwin back to 3.0.7-1 from the latter release and X.11 then
works fine again.

To quantify the problem, I timed these using the BeyondCompare and kdiff3
file diff programs (which really show up the issue being very CPU intensive)
comparing two versions of a source code directory containing 6 largish
files, all differing:

Time to open the window and display the directory:

BeyondCompare with cygwin 3.0.7-1:	2 seconds, 	with cygwin 3.1.0-1:
18 seconds.
Kdiff3 with cygwin		4 seconds 	with cygwin 3.1.0-1:
15 seconds.

Time to open a tab and display the diffs for one of the files:

BeyondCompare with cygwin 3.0.7-1:	1 seconds, 	with cygwin 3.1.0-1:
8 seconds.
Kdiff3 with cygwin		5 seconds 	with cygwin 3.1.0-1:
13 seconds.

Firefox is not so bad, but still slower. The time to open the window and
display the default home page is about 2 seconds with cygwin 3.0.7-1, 5
seconds with 3.1.0-1. And the problem that I reported below relating to slow
response trying to type into Firefox's address bar is not showing up with
this build. Either that was introduced later, or maybe it was some side
effect of the upgrade path that I took then?


> -----Original Message-----
> From: <> On Behalf
> Of Ian Puleston
> Sent: Friday, December 27, 2019 4:39 PM
> To:
> Subject: X11 forwarding extremely slow (unusable) after Cygwin upgrade on
> Dec 23rd
> Hi,
> I have a Cygwin installation on a Windows 10 PC, on which I use X-Windows
> to access a Linux development/build server. All has been working great for
> long time, until I rand a Cygwin upgrade on December 23rd, and after that
> X11 has become pretty much unusable. I normally launch the terminator
> terminal emulator locally under X, in that I "ssh -Y" to the Linux
machine, and
> then on there I use a number of GUI apps, mainly SlickEdit and
> BeyondCompare, which run remotely via X11 forwarding to display in
> windows on my local Windows PC. But since this Cygwin upgrade all of those
> remote X11 apps are now extremely slow to respond to anything - about 10
> to 20 seconds to action any key press or mouse click.
> The two machines are on the same Gig-Ethernet LAN segment so there are
> no latency issues there (ping response time <1mS).
> To take any possible problems with those components out of the picture, I
> tried instead launching xterm under X11 locally, and from it launching
> after ssh to the Linux machine, and again I see the same symptoms with
> - trying to type into Firefox's address bar takes about 10 seconds per
> I had previously updated my Cygwin installation on March 3rd 2019, so this
> problem has come in sometime between then and now. I've just spent a very
> long day and a half rolling back the changes as much as I could (by
> the setup.ini and re-installing previous versions from the local download
> cache) and eventually managed to get a working hybrid with enough
> packages rolled back to make the difference, and now X11 is working fine
> again.
> I also tried a new clean install of the latest Cygwin, after deleting my
> and removing all but the bare minimum from .startxwinrc, and do see the
> problem with that too.
> Is this any known problem? I couldn't find anything obvious in the email
> archives.
> Ian
> --
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