I have recently been trying out tmux on my server as a replacement for byobu (screen). When I started editing some code on vim, I found it very laggy when I held on to j and k to scroll up and down.
I thought it might be because there was something wrong with my connection but I tried editing the same file on vim without tmux and I found it much more responsive with almost no lag.
So, is there anything I can do to make vim on tmux not lag?
It sounds like you are using tmux over ssh. If so, you may want to give mosh a try. It is built on top of ssh but with many advantages over ssh. One of them is:
It worked much better than over ssh in my experience.