where to start

would be great if some experienced drivers/instructors could give me their opinion on this matter please .

so today I had my third day of training and I am very happy with my progress however I am a little bit confused by my instructor .

so I was doing 55mph on a dual carriageway with 60 mph speed limit and a roundabout waiting for us at the end , my instructor told me to turn right at the roundabout and at that point speed limit reduced to 40 so after checking my mirrors I adjusted my speed then i started to see SLOW DOWN signs and yellow paintings and SLOW DOWN marks on the road at which point I could clearly see the roundabout so again I checked my mirrors , indicated to the right and positioned myself on the right lane to turn right at the roundabout and reduced my speed to almost 15-20mph and changed to the 5th gear to prepare myself for the roundabout and everything went really well in my opinion .

however my instructor told me off again for slowing down way too early , he said I shouldn’t have reduced my speed to 15-20 that early , I said but there were clear signs , yellow paintings and also marks on the road telling me to SLOW DOWN, to which he replied no you should have ignored that , those signs are for those who are going fast and they need to slow down but you were already driving slowly so you didn’t need to slowdown even more .
and I was thinking to myself , first the speed limit changed form 60 to 40 and I adjusted my speed , after the speed limit reduction SLOW DOWN signs appeared on the road , how could I be doing the wrong thing by slowing down .

I just don’t get it .