First, I'd like to show you
this, and it solves the problem of Virtue's bulkiness.
Second, I think you assume that Virtue's beam only goes in straight line? I'm not really good at explaining, so I think you better rewatch ep 8,where Virtue sweep out whole platoon in one shot. That's it. Even if you dodge, he just need to turn a small degree, the beam would go a large range. So, the shooting speed problem is solved.
However, Virtue isn't designed to kill one MS. Its purpose is to take down big and station target, or large army in formation that isn't easy to split up.