It’s not that mouse pointers today are
normally laggy; they aren’t, as you correctly say. But when a modern system is very busy, it’s normal for the keyboard and mouse input to lag
too, but that
wasn’t normally the case with older systems, since human input peripherals were typically given their own special higher priority interrupts or whatever.
This was of course done because the systems back then were so slow that they would be in danger of regularly dropping key presses or clicks otherwise, which would have turned people off these newfangled electronic typewriters. But the result was that the interactive experience with respect to latency was given great importance in the design of those systems, unlike today, when it is taken for granted that people will accept whatever laggy system they are given, and will just be directed towards a CPU upgrade if they complain about it.