When I playback a video and it renders, it only renders one piece of clip

Im not sure really how to say this, but every time I let aftereffect render or do something, it always renders half or a small part of my clip. I want to remove a background from this 13-second clip I have. So I roto brushed the people in it that I wanted to keep and let it render as I did it. When I play it back, it is very slow and renders a perfect part of like 1 second of the clip. I am somewhat new to after-effects (like something), so it would be nice to get a step-by-step guide on how to render playback and export. Maybe I'm doing something wrong. My settings are 4k, square pixels, full res, and 30.0038 fps (Idk, thats what my video is). FYI, I haven't yet roto. I did this three times, and I had render problems, so I want an answer before I start again.