The processor in modern Mac Minis is soldered in, so is almost impossible to replace (unless you have specialist equipment and good skills). An SSD would improve loading times, but wouldn't affect the performance of programs. So it rather depends upon exactly what you mean by "Slow Mac Mini"; they are, by nature, slow. I have an i7 Mac Mini and I would describe it as slow, especially graphics performance. But it makes a nice little server and media streamer.
Your best upgrade is to sell the Mini and buy a more capable computer.