How to speed up a slow rendering project with high resolution images (8000x6000x24bit)?

As noted, you aren't giving nearly enough details to actually give a precise response.

But also as noted, the wiser process is to batch the clips in Photoshop Lightroom or whatever to no more than 3,000 pixels the long side for 1080 sequences, 4,000 for UHD.

As that simply works. Being practical.

 Well, at least we can assume you are using a PC. Is it a laptop. 

I'm not sure what the answer is for you. Whenever I've put in files with ~ 6000/* pixels, I've noticed PP doesn't like it. So I to a mass export from PS (or Lightroom) with max size more like 2000/*, then on the odd photo I can get a bigger export if I need the extra size.

Not all things use the GPU in PP, though changing frame size does. But your machine will not magically light up to 100% usage of your various items. Machine optimization is a way to eek out some more performance.

What sequence settings are you using, export settings? Any more hardware details.

With the amount of info you've given, I'm going to guess and say... make the images smaller?