How to mask a render with isolated objs with alpha in an image?

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So I need to tell Blender: render what's on the 2nd image, BUT do NOT waste precious time rendering where the other 4 donuts in the 1st image will show (once I overlap them). In another words, I need to use the 1st image with alpha as a mask to NOT render on the 2nd image. Once done then I can use this nodes setup to merge them:

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I need to do this just to reduce the workload on my old computer. Otherwise it has proven to be too much to handle at once. Of course, the actual rendering I'm aiming for is much larger than this little images here.

I am hoping that by not needing to render where those front donuts will appear might just be enough not to freeze in the process.

Thanks in advance guys,

DPC