Weird response when typing in lLeft part of Web IDE

I've always noticed that the left side is a little flaky

On the Pico, or on the original board too?

As @allObjects says, if you've got code that takes a long time to run, eg:

setInterval(function() {
  for (var i=0;i<10000;i++);
}, 10000);

You'll find that Espruino can be quite laggy. Ideally you'd split your code up into small tasks that each execute quite quickly, then the command-line will be a lot more responsive.