I've given up on streaming Prime Video on my Mac - I have an old iPad 3 and an iPad Air to go with my ATV, and either iPad's streaming via Airplay looks SO much better than anything on my Mac. That, and my Mac always chokes sooner or later on an HD video while my iPads never do.
I've kept my iPad 3 just for email monitoring and Airplay for Netflix and Prime video - the thing is a tank but it works great! My GF has a Roku for Prime video and it works great too. Give up on your Mac for Prime video - the future costs of therapy aren't worth it...