Bạn rất chậm.
Bạn là rất chậm?
Yeah, I wrote "Bạn rất là chậm" and was marked incorrect.
là here emphasizes the following chậm and I think it should be accepted. it doesnt act as a copula linking tôi and chậm, which would be wrong.
Bạn thật chậm chạp vẫn được nhé
you should not have been downvoted because "bạn thật chậm chạp" indeed means you're very slow.