Even though their CPUs are insanely fast, the real power of mainframes is in their IO. The amount of data you can push through those machines is absolutely mind blowing.
Mere mortals such as me, have to make do with cloud-based clusters where the IO is distributed across multiple machines connected by very fast interconnects, but they don't behave as a single machine, nor have the IO always accessible at local bus speeds.
> Mainframes phone home immediately if they detect problems and an IBM service person will be on site the same day to fix it.
The joke usually went like this: technician shows up to fix the machine, operator says "We didn't call you", to what the technician answers "You didn't. Your computer did"