Slow load time of bash in cygwin

In my case that was windows domain controller. I did this to find the issue:

I started with a simple, windows cmd.exe and the, typed this:
c:\cygwin\bin\strace.exe c:\cygwin\bin\bash

In my case, I noticed a following sequence:

    218   12134 [main] bash 11304 transport_layer_pipes::connect: Try to connect to named pipe: \\.\pipe\cygwin-c5e39b7a9d22bafb-lpc
     45   12179 [main] bash 11304 transport_layer_pipes::connect: Error opening the pipe (2)
     39   12218 [main] bash 11304 client_request::make_request: cygserver un-available
1404719 1416937 [main] bash 11304 pwdgrp::fetch_account_from_windows: line: <CENSORED_GROUP_ID_#1>
    495 1417432 [main] bash 11304 pwdgrp::fetch_account_from_windows: line: <CENSORED_GROUP_ID_#2>
    380 1417812 [main] bash 11304 pwdgrp::fetch_account_from_windows: line: <CENSORED_GROUP_ID_#3>

    etc...

The key thing was identifying the client_request::make_request: cygserver un-available line. You can see, how after that, cygwin tries to fetch every single group from windows, and execution times go crazy.

A quick google revealed what a cygserver is: https://cygwin.com/cygwin-ug-net/using-cygserver.html

Cygserver is a program which is designed to run as a background service. It provides Cygwin applications with services which require security arbitration or which need to persist while no other cygwin application is running.

The solution was, to run the cygserver-config and then net start cygserver to start the Windows service. Cygwin startup times dropped significantly after that.

As you've noted in your answer, the problem is Cygwin's bash-completion package. The quick and easy fix is to disable bash-completion, and the correct way to do that is to run Cygwin's setup.exe (download it again if you need to) and select to uninstall that package.

The longer solution is to work through the files in /etc/bash_completion.d and disable the ones you don't need. On my system, the biggest culprits for slowing down Bash's load time (mailman, shadow, dsniff and e2fsprogs) all did exactly nothing, since the tools they were created to complete weren't installed.

If you rename a file in /etc/bash_completion.d to have a .bak extension, it'll stop that script being loaded. Having disabled all but a select 37 scripts on one of my systems in that manner, I've cut the average time for bash_completion to load by 95% (6.5 seconds to 0.3 seconds).