What is the best timeline to use for my workflow?

[John Br]

Hello all, this is the second time I have tried to get answers about this, hopefully someone knows.

I am doing chroma key work. In order to get the most data I can for the key I am recording in the highest detail avilable to me, 4k (UHD) at 4.2.0. But I don't need (or really even want) to upload at 4k resolution (slow internet speed, moderate GPU) So I render at FHD. I use stock imagery in my videos, a lot, and when I work in a 4k timeline it does not give me the flexibility to crop zoom on the photos as much as when I do in FHD, because I face pixelation at a much lower crop percentage, (due to the resolution). But since I am rendering at FHD I don't know if it makes any difference. 

Can anyone guide me as to how premiere handles a workflow that is different than the render size? 

I have found this:

Working with 4k footage in a 4k timeline then rendering at FHD seems to render about 20% faster than working with 4k footage in an FHD timeline and rendering at FHD when rendering.

Thanks to anyone that can give me insight.

TL;DR how does premiere handle subsampling? How do I optimize for render times and chroma work between 4k and FHD?

[John Br]

I have run some test patterns, the same image at 250% scale, both rendered as FHD, but one in a 4k timeline and one in a FHD timeline, the difference is HUGE. I don't mean literally, I know the sizes because of the timeline at different resolutions while keeping the same percentage increase in size relative to the original image, but the render quality differences are massive! 

Can someone help me understand this and what it means for my workflow? (recording at 4k for maximum data and rendering in FHD)

1080 Sequence output

4K Sequence

Those vertical lines on the second one are not rendering to much more than a blur.