I have never seen a computer that did not experience a significant increase in speed. I put one in a 7 year old laptop and it was so much faster, it felt like I had upgraded the entire thing.
The most likely reason you did not see an increase in performance is that your hard drive controller is not capable of utilizing the the increased speed available with an SSD. This is often the case with pre-built brands. The profit margin is so slim on computer stuff that the mfgrs build to the spec. Example not using real life numbers. If the HD the computer is built with can do 300mbs a second, in order to save money, they put a controller in that can only move data at 300mbs a second. If you later go back and install a drive that is capable of 3000mbs you don't see the increased speed because the controller is incapable of working that fast. If your hardware can use the increased speed, I assure you, the speed increase you get moving from a mechanical hard drive to a solid state drive is very noticeable. But don't just use boot times, an SSD will improve the computer in most aspects