That's not been my experience at all - I can't say I've ever seen tmux perform noticeably worse than screen, but under extreme load tmux always seems to cope be

That's not been my experience at all - I can't say I've ever seen tmux perform noticeably worse than screen, but under extreme load tmux always seems to cope better. e.g. dumping 250k lines takes screen over a minute, while tmux will be done in under 3 seconds and be more responsive while that's happening. It's nice to be able to ^C something spammy and not have time to go make a cup of tea before it responds. I have experienced lagginess with tmux, but this has been more down to VTE and/or mosh performing particularly badly with large terminals. A fullscreen 266x188 tmux session on urxvt is massively smoother than with Terminator/gnome-terminal/Terminology/etc, and with remote sessions ssh is similarly massively smoother than mosh. Kind of disheartening given today's ridiculously powerful hardware.