Hi,
From my limited experience, my suggestion is that you need to look at how much RAM you have. I suggest you look at getting 64 GB or more. Also the speed at which you can read the data will make a difference, so the faster the hard drive the better, ie NVME > SSD > HD.
The format at which you data is in will also make a difference.
This post might help understanding the background but in short get the openfoam data into vtm format it will save processing time.
and https://www.youtube.com/watch?v=AoUygiAk4fc
Careful of Index, it is not a silver bullet that will solve your volume rendering problems. I have not be able to get one of my openFOAM simulations to be able to render with index, I have tried with a snappyhex mesh and a cfmesh mesh. I keep getting a topology error. So if I need to do a volume render I still resample to image and then volume render, however to do this with a large data set needs lots of ram. My suggestion is to clip your data down to a subset then sort out your colouring and opacity. Then apply that to your data set to get the image that you want, using a python script to do it will be better.