I first noticed incredibly slow rates across my LAN a while back, but lazily switched tools and forgot about the bug - assuming it'd be obvious and get fixed eventually. Oops.

I finally decided to run some loopback tests to report it properly.

ncat -l --recv-only 5000 > out.bin
ncat --send-only 127.0.0.1 5000 < in.bin

Transfer times (via a wristwatch) on Windows 7, for an arbitrary a 9MB file, with matching versions of ncat.
6.40 sending to 6.40 - Instantaneous
6.45 sending to 6.45 - Four minutes!
Every released version thereafter* (until 7.50) is also slow.

If I mix versions to tease apart whether sending or receiving is the bottleneck...
6.40 sending to 6.45 - Instantaneous
6.45 sending to 6.40 - Four minutes
So 6.45 can receive quickly, but it is slow to send.

• 7.60 was not tested due to issue ncat: nsock_loop error 10038 #978