I don't understand why a horsebox should have been holding up the traffic to such an extent. On straight roads, our Disco & Trailer combo cruises at its legal limit very happily.
When we ran a 7.5T lorry, that would also cruise happily at its legal limit.
I do go a lot slower on roundabouts and corners though. There are 3 people offering advice on that point when I get it wrong. Fortunately, only one of them can talk.
Fortunately, our trips through the fields of Lincolnshire have not been too badly affected by tractors when we take the horses that way. In my experience, most have pulled over once the queue behind them became substantial.