I also did something similar in the past. I had a 2008 MacBook and I got its HDD upgraded to a 128GB SSD. I also got rid of the Mac OSX and installed ubuntu.
It changed the computer. It is still working. I agree with some of you saying that the actual bottleneck is the hard drive. For me the second one is memory and CPU. I am not sure I can choose between the two, as if any of the two are old, you will notice it very much.
The only problem I have now is that the computer is so old that it is starting to fail randomly. I suspect the main board is dying. But hey! It’s a working computer that is mora than 7 years old!