By default, tmux imposes a 500ms delay on presses of the escape key. This is in case the escape key is used as part of a tmux key binding.

I don't use the escape key for any of my tmux key bindings, but I do use the escape key quite in other contexts, like Vim.

The 500ms delay on the escape key being registered in contexts like Vim is annoying at best. This delay can be removed by overriding the escape-time option.

set -sg escape-time 0

Adding this line to your ~/.tmux.conf file will set the delay to 0ms. The -s and -g flags set the option for the server and the global server contexts respectively.