That's a good question. I don't think it's a matter of how much you have installed... but more about how much you have running at one time. Some programs may start background processes that you are not aware of, and the more things running may eventually begin to cause lagging, depending somewhat on your RAM. Also, some programs may have coding issues creating "memory leaks" that can cause extreme slowdowns in your system. If you haven't learned about it yet, you will find top to be a good command line tool to help you monitor system resources.
If you mean "operating fast" (over time) as compared to Windows... I think Ubuntu will be a hands-down winner!