bicycle_rm has it right, Neil_18. As much as we like to help folks around here, we don't regard making thiings too easy for you to be any help.
You'll hate me for this but before I left school 53 years ago, I would have had to go through a dozen books to find one answer in a paper then do the same for all the others.
These days, you've got it made and Google is your best friend. The answer is in there somewhere and it might even be here in Tom's but you have to do the work and find it.
Anyway, I always thought the ideal speed is the fastest you can afford but I'm not taking an exam.
Unless your teacher specified an ideal rate per computer, the ideal speed would depend on several variables. Everything comes down to how much money is time worth and how much does a given rate cost.