Claude, I can only tell you what has worked well for me. I primarily work with Sony FS7 XAVC files at 240mbps.
On my PC system, I’m using an Nvidia GeForce GTX 970, an i7-5820k CPU with 32 gigs ram. My media lives on an OWC Thunderbay Thunderbolt2 raid which connects to my PC through an Akitio TB2 to TB3 adapter. With this setup I am able to work without issue in full res with Luts attached with all of my 4k media.
I guess with your imac you’ll need to know what the data rate is for your media and then determine if your media drives are fast enough. in addition you’ll need to determine if your graphics card is powerful enough to make use of OpenGL. All of this may require doing some tests on your system with 4k footage.
If you have the right components there’s no reason why you can’t work smoothly with 4k media and avoid the very time-consuming proxy creation workflow.
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