CPU is basically the engine or the brain of your system. The CPU carries out any task. If I hit ENTER, the CPU tells the computer what to do with that information. Depending on how fast your CPU is, the faster it will complete that task. Basic stuff like typing or surfing the web doesn't use a lot of Processing. However, recording or rendering or editing (manipulation) of images requires more "information" so the CPU needs to be faster or you will be sitting there waiting.
For example, if you give a package to an elderly man to run two blocks to deliver vs a younger man..which will complete the task first?
When you record there is more things involved. You have the midi, tempo, vocals, etc....All of this is put on a Bus which takes it to where it needs to go. If that information gets there late you will have some lag (latency). With a fast CPU that latency will be less. Plus time is money. If you are waiting for your computer or iPad to render the sounds, thats a lot of time wasted. I remember back in the day I was using a PC with Windows 98, the thing kept crashing because the PC either lost the info while on the BUS or just couldn't take all that info at once, so it shut down.
So when you are recording, the more information (instruments) you add the faster your processor needs to be. If you want to record on iPad its best to get the fastest that out there. If you have an idea you want to put down, you shouldn't have to wait for the iPad to load. I hope that makes sense.