- No computer will ever be as fast as you want it to be.
- Even if you do get the fastest, most powerful computer you can possibly afford, tomorrow or next week you will decide to expand your problems to be just a little larger, because your computer is now big enough that the current model is just too easy, and there is more complexity you want to add. And the week after, just a little more. Soon enough you will need a bigger, faster computer. AGAIN.
The latter is the crux of what I have always called John's law - that your problems always expand to be just bit too large for your current computing environment. (I'm sure others may know the same thing from some other source, but I first wrote about that law at least 40 years ago.)
As for not taking too much time, too much is entirely your personal definition of what is too much time. I have long accepted that for some problems, I must set my computer running when I go to bed, and it will be waiting happily for me in the morning.
There are other options of course. You can always rent server time, on machines that will put anything you can buy to shame. I would note of course, that the largest Mersenne prime was recently found, by a fellow who was willing to spend roughly $2 million of his own money on exactly that, renting server time.
