Slow Terminal on Sierra
After upgrading to High Sierra, I also noticed that there is some lag in Terminal.app after pressing Enter. I was able to fix it by doing this:


Go to Preferences > Profiles > Shell
In the "Run Command" box, write login -f &lt;username&gt; (replace &lt;username&gt; with your username)
Uncheck "Run inside shell"


I suspect that Terminal.app tries to integrate with the shell somehow and running bash indirectly fixes that.