Stem elongation and leaf production should stop even on autoflowers at some point while plant is flowering.
if you refer to bud development being slow, it's probably just focusing on fattening up. It seems slow but many things are happening constantly. plant looks healthy, so i wouldn't worry too much. temps and RH look fine. Canopy is still strong at day 51 - keep it strong. density will improve as it ripens. Long hairs shows a lot of ripening left to occur.