We have a custom mod that limits the megs per user per day.
Other then that your server shouldn't slow down unless it can't handle the load. You may just need to upgrade your server load handling ability.
Also make sure and block download bots from stealing your content.
http://www.dwhs.com/hostingforum/viewtopic.php?t=1114
And password sharing obviously will drain your server as well so blocking that and hotlinking will make a big difference.