You can try to close background apps, put them to sleep/deep sleep, disable/uninstall apps you don't use to improve your playback experience.
Unless you're watching it on a 4k TV or Projector, it would be a great experience.
Thanks for sharing your experience here with your beloved Samsung Galaxy A12 having encountered extremely poor video playback after downloading high quality resolutions videos and we are sorry to hear that. I hope with the replies given by our dear Samsung members here, you will be satisfied. If not, please call us on the below Samsung Customer Care support channels.