Found it. Upstream PR: https://github.com/tmux/tmux/pull/4013
-- Romain Francoise <rfranco...@debian.org> https://people.debian.org/~rfrancoise/
- Bug#1073981: tmux: entering copy mode is very slow with l... brian m. carlson
- Bug#1073981: tmux: entering copy mode is very slow w... Romain Francoise
- Bug#1073981: tmux: entering copy mode is very sl... brian m. carlson
- Bug#1073981: tmux: entering copy mode is ver... Romain Francoise
- Bug#1073981: tmux: entering copy mode is... Romain Francoise
- Bug#1073981: tmux: entering copy mo... Romain Francoise
- Bug#1073981: tmux: entering cop... brian m. carlson
