apply glsl shader LUMA on downscaled image

[geextahslex]

Hi, this is a performance related issue. I watch 4k on 1080p. I want to use a glsl shader with the HOOK LUMA but on the already downscaled image, like OUTPUT. OUTPUT is downscaled and costs less performance than LUMA and I need LUMA because it doesn't change the colors like OUTPUT. I hope this makes sense. Can the pipeline be changed? Is this somehow possible? ^^'

Thank you :)