I did similar tests, by downloading the same file from servers hosted at different providers, and the bandwidth was always very slow, except when I changed the HTTP port from 80 (the standard) to an arbitrary chosen number! This change alone made me able to use the full bandwidth provided by my physical link! It was the proof that my ISP was running some kind of traffic shaping, which is a shame.
More information if you read French: https://twitter.com/ngrilly/status/756453318113783809?s=19