512 Kbps vs 2 Mbps streaming video

Depends on the phone and the screen quality and size. If you have a phone with a tiny low res low quality LCD screen (perhaps you bought the $20 tracfone) then it will probably not be noticeable. But if you have a phone with a gigantic 4k OLED screen, you may be able to notice things are a lot fuzzier or blurrier or just unwatchable.

Tablets are a similar deal, probably not on a cheap one for $20 on special with a 6″ screen, probably will on a huge 12″ ipad pro or similar.

512kbps is typically for 480p resolution or below, 2mbps should get you at least 720p or over compressed 1080p.