I have a running Windows 98 PC with P3 550MHz and SSD connected via IDE adapter. The drive is a bit out of place, but it’s legitimately the fastest booting, most responsive computer in my house. Its only speed issue is that it can really do just one thing. Any heavy task (including large file/network operations, apparently) will render the system almost unresponsive until it’s finished.
I wonder how the CPU in the SSD controller compares to the one in the PC.
That depends upon the type of SSD. There’s compact flash which is natively IDE, and the controllers are extremely weak, then there are modern TLC SATA drives where the controller would probably be a bit more powerful than the machine’s CPU.