I'm also considering xterm.js, but do you see any performance issue using it?
Also, can it be easily customized?
I don't feel that performance is an issue, it depends on what you mean by performance. Memory consumption is higher than with native terminals. From a speed perspective, I do not see any difference. You have to deal with the same problems as VSCode for example.
My colleagues have been using the terminal productively on Windows for more than a year. It runs stable, the performance is ok. As the autocompleter learns from your input, it becomes more and more personal and tuned to what you are doing. I don't want to miss it anymore.
Cogno is open source. You might want to take a look at it. The master branch is a mess at the moment. I have been rewriting Cogno for the last 4 months. When the current branch is on master, things should be easier.