The Multi-Boot Configuration Handbook

CPU Speed Requirements

Like RAM, your computer's CPU is a resource that each OS monopolizes when running, so adding a new OS won't make an old OS any slower. You might need to consider a CPU upgrade if you add a new OS with substantially higher CPU requirements than an old OS, however.

The CPU arena changes frequently, so if you need to buy a new CPU, you should research what is available at the time you read these words. In general, the very fastest CPUs cost twice or more what a CPU just a few percent slower costs, so the best values typically come in something a bit less than cutting-edge performance. You might also find better value, and possibly better performance, with Intel-compatible CPUs (such as the AMD K6-III and Athlon processors) ...