I don't understand that. Surely, if someone in front is travelling beyond the limit and you are inside the limit then the gap between you will be increasing. Even if you were both at the same speed you must be at a safe distance so if that gap is increasing with every metre travelled then that safe braking distance will also be increasing. So why should the car in front slowing down to match your own legal speed cause any comment from you? Your own progress is unaffected and the gap between you is still there.
Of course, you may be complaining about someone who is travelling within the legal speed but reacts to a warning beep as they check that they are indeed inside the limit. That is quite common in areas where the limit changes frequently as it is very easy to be caught that way. That can be annoying but a follower driving correctly has enough space to react, especially as the unexpected slowing could be caused by an unseen incident in front and nothing to do with a speed warning. But such a driver is not speeding at all. Just making sure that they are not a victim of a 3mph infringement.
What I'm talking about is free flowing traffic on a dual carriageway with a 70 speed limit. I'll probably be doing about 65. I'll be passed by lots of folk doing 80... that doesn't particularly concern me btw. Then, when there's some sort of alert, people drop their speed to well below mine.
It's nothing to do with whether I, or anyone else, can react in time. It's not stamping on the brakes.... It's indicative of bad driving where people don't know what the speed limit is, don't know how fast they're going, slow down far too much because their car has gone "bing" as a pavlovian response.
In and of itself it's not a problem but I generally get irritated by people who will flash me to get out of their way because I'm not passing the lorry as quickly as they'd like, then they'll slow me down because they have no situational awareness.
It's a personal gripe and I'm a miserable git who will rightly be ignored.
I do have a problem with speeding in other circumstances though. It's endemic and a big problem imho.