... connected to a mainframe (KDF9, later a DECSystem10). One user with a compute-intensive job could bring the room to a total standstill, then gradually the terminals would come back to life as the workload eased.

As a student in the late 1960s I had a tour of my local Strowger exchange in the UK. I recall the engineer telling us how he could hear subscribers 'tap-dialling' (dialling using the handset rest of a pay-phone to avoid paying for calls) and could get to the rack to clear down the call before they had finished.