i think its pretty unfair that youre always answering everyones questions but no one ever answers yours. to that end i did some research and it turns out that windmills get faster when the wind gets faster but if the wind gets too fast the windmills slow down so they dont break. therefore the answer to "when did all the windmills start to turn so slow" is probably either "when it got too windy" or "when it stopped being windy". hope that helps
you're assuming that they were turning faster before. is there not another possible context where it could be assumed that they were previously not spinning at all