Consistent speed trouble with 100mbps business account

Hey, new to the forum, we've got an excellent wireless router and at one time routinely received most of our bandwidth - 85 - 95 mbps, for the last month we have been getting long connection times and at maximum 50mbps. That is half of what we're paying for, and it isn't cheap. Upload is also slightly more than half - 13mbps out of 20. Any help appreciated.