Please define "Weak CPUs"
It's easy to state mainframes have weak CPUs. But what exactly do you mean when you say that? Modern mainframe hardware supports 96 CPUs, running at 5.2 GHz. Moreover, the mainframe uses an extreme CISC-architecture, which means that compilers have a wide range of instructions to choose from. This is freedom of choice that enables optimization to a high degree.
Finally, the processors support out-of-order execution. If you know how it works, you can code for efficiency and speed by exploiting its specific features. And guess what - compilers are pretty good at doing just that, and constantly being improved, too.
Of course, it takes quite a bit of storage to support this computing power. You need to load a mainframe to at least 70% to make it run really efficient. So you can have up to 3 TB of internal storage on a box.
So, if you think this is a weak processor, then you must know of a much stronger one. I have not yet heard of such a strong processor, but I am sure you'll be willing to inform me about the specifics.
Thanks in advance,