This irritation has recently developed where people finish speaking but this mouths stop talking about a second or two later.
Is there a setting where this can be remedied?

Is your computer old, and/or is it under spec?

PC quite old. This is my third play through and now on level 20 and I have only recently noticed this problem.
It certainly did not exist prior to this.

If it's old, and the problem is caused by hardware timers, which it may or may not be, being a third play through is irrelevant. You could try toggling on/off your high precision timers (look up HPET), but that won't guarantee the problem will go away. Only an upgrade will do that.

I have just checked Fallout 4 and there is no lagging problem there. It's only in this game. Strange that it should only occur now.

Because one game does it, doesn't mean that every game will be plagued with the same issue. The code for each game is entirely different. What I would do first is to remove any and all mods, then completely uninstall the game and reinstall from fresh. Saved games will be kept if you direct the uninstaller to do so. This should rule out any configuration glitches.

Thank you. I think I will do this if the lagging becomes too irritating.

UPDATE: The problem has sorted itself out, no more lagging.