It's, well, slow. Haven't done any measurements, but it's noticeable. To do its work tmux must be basically another terminal emulator, and it seems it isn't the fastest at it. It's still strange, neither a little context switching (tmux is an extra process in the pipeline) nor doing the terminal things should amount to more than a millisecond. I think.
It's extremely noticeable in certain situations on MacOS.
Use a full screen terminal with a 12pt font on a 4K display. Split the tmux window into quarters and pull up some logs in each pane. Now use the mouse to click and drag on the center vertical border; quickly resize it from left to right. The border takes about half a second to catch up with the mouse cursor as tmux repeatedly reflows all the text on the screen.
Different people have different sensitivities. I also don't want to use Gnome Terminal or most other terminals for that matter, because of latency. I also find Windows Cmd window slow. Never done any measurement, but xterm feels right.
