New pty implementation is really slow

[Joe]

I'm using cygwin 3.1.4 on Windows 10.

The new pseudo terminal stuff seems really slow.

For example:
$ time seq 10000
(output omitted)
real    0m23.510s
user    0m1.515s
sys     0m4.483s

If I minimize the mintty window while seq is running, it gets slightly better:
real    0m4.562s
user    0m0.390s
sys     0m1.202s

But when I set CYGWIN=disable_pcon before starting mintty, I get:
$ time seq 10000
(output omitted)
real    0m0.366s
user    0m0.109s
sys     0m0.093s

So the new implementation seems to be over 60 times slower than the old one.