When I watch 1080p videos or any less the quality is so bad... but when watch 4K is good.
This happen when have many action in video like an gameplay of some FPS games that has a lot of camera movement, but if you watch videos like in 4K with little camera movement it's great, an print of the problem: https/prnt.sc/l2wzne
if you want to know GPU is 1070GTX (416.16) and the problem not is my internet.
Does this happen on other devices connected to the internet as well? Could be modem/router problem as well.
Ethernet/wifi driver is up to date? Is anyone else using your internet connection at the same time?
Post the youtube link, eventually it´s the video itself.
Post the complete system config: CPU, RAM, internet connection speed
Try a different browser like edge, internet explorer, firefox, chrome
Monitor an VX248H from Asus, i7 7700, 8GB ram ddr4 2400mhz, my internet is not too fast but I can watch 1080p very well and I made some tests in the PC of my friend is not slow in loading videos and not have the problem that I have (1mb/s download and 500kb/s upload) all up to date and just me using internet. In my phone the quality is great using WIFI.
Used edge, chrome and firefox same problem, oh and I downloaded the latest Adobe Flash player.
Link of the video in the printscreen: https/www.youtube.com/watch?v=JuctMzYY_Lw
is not just in this video but all videos in 1080p.