The nice thing about tmux/screen is that they're more widely available by being separate from the terminal emulator itself. I can use them on any *nix system and with one modification (I use a different prefix than default with tmux and screen, I'm too used to emacs keybindings to let them have C-b and C-a) have the same experience on all of them.
Another positive is that they survive if, for some reason, you close your terminal. I can completely kill the terminal emulator and my tmux session (or screen, but I haven't used it much in years) will still be around and can be reattached.
I've used tmux and vim/nvim on macOS and never experienced performance issues but I'm really curious about your experience, could you share more details?
I have no measurements or anything but if I open a file in neovim within tmux, I get more lag than if I open the file directly without tmux in between. I've seen people on github having similar issues
Another positive is that they survive if, for some reason, you close your terminal. I can completely kill the terminal emulator and my tmux session (or screen, but I haven't used it much in years) will still be around and can be reattached.