nothing is bottlenecking on that system unless something is damaged...
the hdd is slow but not exactly system breaking... i dont have an ssd and my games run just fine. i do have load speeds of 200Mbps but theres not a hugley noticeable difference coming from 100Mb which is where your drive fits...
you would benefit from getting another identical drive and raid 0'ing it but there would be little you would see in games other than lower texture pop.. so no reason to get an ssd just yet...
chances are its a driver issue... or the actual games your playing. if they dont stress the gpu then you may get an over abundance of fps, the problem is when you run in to an area that suddenly increases the stress and dropps the fps you will really...