Here is a simple command to launch an emacsclient frame in Windows 10, which I've added to outlook as a macro and assigned it to a new button.
Shell("c:/Users/user_name/emacs-25.3-x86_64/bin/emacsclientw.exe -c ")
But every time it is launched through that button, the outlook message window becomes non-responsive for 15+ seconds.
If I change that macro to:
i,e, to launch emacs instead of emacsclient, it can finish instantaneously without any impact to outlook window.

If I launch emacsclient directly in a Windows Powershell with this command:
c:/Users/user_name/emacs-25.3-x86_64/bin/emacsclientw.exe -c 
a new frame can also pop up instantaneously without any noticeable impact to anything.

so launching "emacsclientw -c" directly is fast. So is launching emacs through an outlook macro. But launching "emacsclientw -c " through an outlook macro is very slow.

Can anyone shed some light on why this is the case?

(BTW, "runemacs.exe --daemon" is part of my Windows startup script.)