10 times faster than a virtual call is still ridiculously fast on any general purpose CPU made in this century. We often forget how insanely fast they are (and at the same time, I have no idea how we manage to add as much bloat to certain stacks that they make the processor struggle).

Like, I’m fairly sure you could have decent latency if your callback function for onMouseMove made a local network call to another process.

Also, how fast do you think download progress should update? Animating it to the next keyframe every second is much more than enough.