I wonder for me at least, why Instagram Reels is super blurry compared to TikTok, like since they should have the all the same conditions except for #5.
Instagram uses DASH (chunked, adaptive streaming) and starts playing with a low res, and aggressively compresses video. Tiktok's api just returns a bunch of different CDN urls for different MP4s of the video (HQ often being 576P CRF24), and the client loads whatever bitrate it has detected it can load without hiccuping.