You should do a compression or leak down test, to test if your engine is still holding proper pressure. If your timing belt snapped a while back, and you noticed a difference in performance; then you probably bent or chipped a valve. Check your engine light codes as well, narrow down the issue, and go from there. About your coolant lines, did you bleed the coolant to relieve any air bubbles when you did the timing belt and water pump? I believe there should be a bleeder valve to do so, correct me if I'm wrong guys.