Tmux output is slower when vertical splits exist: why?

Question

When I use Tmux and split the window into panes, I notice that programs are quite a bit slower to print out lines of text when there is vertical split (two panes side by side) than when there is a horizontal split (panes stacked). Does anyone know why?

Solution

See this answer (last point in particular). Basically, tmux has to do a lot more cursor movement and other extra work to emulate a reduced-width terminal than it would have to do to emulate a reduced height terminal. With a full-width terminal, tmux can just let the terminal emulator it is running in handle the line wrapping, while it has to do its own line wrapping to emulate a reduced-width terminal for side-by-side panes. Each cursor movement involves outputting several bytes (possibly 7 or 8 bytes or more) of escape sequences to tell tmux's controlling terminal to move the cursor, and when lines of text are being rapidly output to the screen and scrolled around, this can add up to a perceptible difference.