Course press is getting too slow

As our site grows we have many subscribers. Over 5000 so far taking a course at least once. As it grows the site gets slower and slower.

For example if you click any normal link in our top menu header its normal speed (fast). But as soon as you click "Course", or do anything with coursepress (eg. login), it slows down and takes like 6 seconds for a page to even come up. In a month it will take 10 seconds at this rate. What can I do to speed it up?
http://www.sidelinesportsdoc.com/usclubsoccer/