Is there a performance issue with GNU Screen, Tmux, and other similar development tools on the MacBook Air?
The basic question is compared with what?

Yes it will be slower than a new MacBook , iMac etc but we programmed with those tools 10 years ago on slower machines.

All you can say is it slow for you and if so you need to get a faster machine:)

Which is why I have always programmed on a desktop machine as they are faster than laptops of a similar vintage