Super slow tracking mask with a simple video

Vertical video from a phone often has a variable frame rate and is usually 1920 high at a minimum. From my phone, it is 4K. MP4s are easily corrupted. You should transcode to a standard production format using the Adobe Media Encoder and change the interpretation to a standard frame rate after importing by selecting the footage in the Project Panel and using the File menu to Interpret the footage. Set the frame rate to the nearest standard frame rate.

This footage should be interpreted as 29.97 fps because that is the closest standard frame rate.

If you put the footage in a standard HD comp, it will have to be scaled, even if the original footage is 1080 X 1920. The black on either side of the layer doesn't matter because there is nothing there for the Mask tracker to track. It won't slow you down.

If you pre-composed the layer and moved all attributes to the new comp, the mask tracker will look at all of the pixels in the layer, but the tracking should not slow down. If you pre-composed a stack of a bunch of layers before running the mask tracker, then it will slow down because AE calculates every pixel on every frame on every layer.

Mask Tracking will only track one mask at a time. You will have to track each of your four masks individually.

The most likely cause of the slowdown is the MP4 file and the variable frame rate. Transcoding to a movie file should help that. Fixing the variable frame rate problem through interpretation should also help. 

If you just want masks, Mocha AE will do a better job and give you more options than AE's mask tracker. If you are tracking faces, then Pre-composing will help. The larger the mask, the longer it will take to track. 

I don't know why your system is slowing down so drastically, but it probably has a lot to do with the MP4 format. 

If you explain your design goals, we might be able to point you to a more efficient workflow. 

How big is the mask? What is the source of the video (MP4s can be problematic)? You didn't give us much to go on. It could be a GPU issue. Screenshots? (Embedded using the toolbar, please or drag to the reply field so we don't have to download them)