Not playing in real time preview ae

you are watcing your machine building the ram preview of your video, after this is done it should play in realtime, the way to know it's ready is when there's a full green line at the top of your timeline panel.

HOWEVER (and this is important), Ae is not an editing software and it's not built to play full res permantently. as you get proficient with it, you will find that 90% of the time you are working in half or less resolution, skipping frames and with motion blur, grain and other render intensive features turned off, only going full res ocassionaly to check frame looks. Slow, choppy playback is not necessarily a sign of a problem, but the way it's supposed to work.

Two questions, what are your computer specs and why are you using 2020? 

I'll try Ty!

If your Preview is not real-time, try lowering the Composition Resolution and/or skipping frames.

If you're currently previewing at Full, try Half, Third, or Quarter.  This trades picture quality for faster render time.

If you're currently previewing at the full frame rate, try skipping 1 frame or 2 frames.  If the Composition Frame Rate is 30 frames per second, then you'll be getting 15 frame per second previews if skipping 1 frame per second and you'll be getting10 frame per second previews if skipping 2 frames, but it is still real time (hopefully).  The skip amount is set in the Preview panel.  This trades smooth playback for faster render times.

If the Composition Resoltion is set to "Auto", you lower Composition Resolution simply by lowering the Magnification (zooming out).  So, press comma to change from 100% to 50% (sets the Comp Resolution to Half) or 50% to 25% (sets the Comp Resolution to Quarter) and then press Shift Nomberpad 0 to skip 1 frame.  This assumes you're using the default Preview options. 

When you're ready to see your animation at full resolution and full motion, render the Comp at settings that are easy for your computer to play back.  I like to customize the High Quality Output Module Template to use ProRes 422 Proxy for sample renders with the Render Settings Template set to Best Settings, but sometimes I also create a custom Render Settings Template that uses Half  Resolution instead of Full Resoltion.