4k capability?

I understand the alpha supports 4K at 30hz , and when I boot up I Display at 4k. But when I do gaming , I can’t do a thing because it’s so choppy for audio and video. I’ve also watched tutorials and persons said you need a USB-C Hub with Display port out in order to render 4k. I don’t use that setup and just use hdmi out. And as I said, it renders 4k to desktop. Does anyone know why it can’t process things in 4 like simple games , or why others say you Display port out for 4k?
Thanks.