High Sierra - Terminal lags if background opacity is set - Ask Different
apple.stackexchange.com/questions/299982/high-sierra-terminal-lags-if-background-opacity-is-set
What Jamie wrote
This bug in High Sierra is really bad for terminal. I have a MacBook Pro (Retina, Mid 2012) and with opacity set to 90% on terminal windows they are unusably slow. If I don't use transparency it works fine. Oddly enough when the window is in the background it seems to go faster. 😞