There actually is a known issue with using vim in tmux on macOS. From the Alacritty README [1]:
> This appears to be an issue outside of terminal emulators; either macOS has an IPC performance issue, or either tmux or vim (or both) have a bug. This same issue can be seen in iTerm2 and Terminal.app. I've found that if tmux is running on another machine which is connected to Alacritty via SSH, this issue disappears. Actual throughput and rendering performance are still better in Alacritty.
> This appears to be an issue outside of terminal emulators; either macOS has an IPC performance issue, or either tmux or vim (or both) have a bug. This same issue can be seen in iTerm2 and Terminal.app. I've found that if tmux is running on another machine which is connected to Alacritty via SSH, this issue disappears. Actual throughput and rendering performance are still better in Alacritty.
[1] https://github.com/jwilm/alacritty#faq