There are so many variables here.
For example are you watching YouTube, streaming a video site, watching on board video?
The video format is important to give you the best advice.
Generally, the stock video play can be replaced with another from the Play Store:
Changing your browser can also help.
Try Chrome with YouTube.
After that you need a fast download and a light phone - by which I mean clear history/cache and especially pictures saved on your phone.
Photo's are best sent to cloud storage like Dropbox.