High Sierra - Terminal lags if background opacity is set
So, for completeness' sake since there doesn't seem to be a better answer as of yet, I'll add my solution.
If your Terminal.app started showing the behavior outlined in the question and you have modified the blur/opacity settings, turning these off will restore previous performance. To do so, open the terminal settings (⌘ + ,), switch to the Profile tab and click on the background color picker next to color &amp; effects. Here, set the faders for opacity and blur to 100%.  



If someone finds a way to enable transparency again without trashing performance, I'll be happy to accept their answer.