I'm not infrequently surprised how people don't spot that something is orders of magnitude slower than it should be or pick the wrong architecture because they can't or won't do simple mental math to work out very roughly how long it should take to move some data around in memory, ssd or over the network or perform some simple computation on it.

I'm having trouble believing that people who think a CPU can do thousands of instructions per second will do well at this (or reasoning about the memory hierarchy).