It's a rule of thumb, not a rule of fist. But actually, it's not uncommon that you might have enough caching (or an in-memory DB) that you appear to be maxed o

It's a rule of thumb, not a rule of fist. But actually, it's not uncommon that you might have enough caching (or an in-memory DB) that you appear to be maxed out on CPU - but because of memory latency, overhead between the DB and the network, etc. you can still get a fair number of operations for "free". Measure everything, but know where your starting points are and what to try first.