Different words, but close to what I was saying.... (RAW data is NOT highly interdependent)
RAW to JPG - parallel process. The more cores the better.
In theory, using a GPU with multiple cores (There are NVidia chips with 512 CUDA cores) will speed up rendering of images.
The raw data is interdependent, but demosaicing the images isn't the hold up in Lightroom for just about anything. Raw to JPEG is largely irrelevant since turning a raster image into a JPEG is only done for previews and exports, and takes very little time. Turning demosaiced data into raster data *with all your corrections applied* takes some time and is not highly parallelizable.
Of all the things that are inefficient in LR, the raw processing pipeline is the least. It's actually pretty efficient. Now, handling huge numbers of previews and putting them up in a grid, doing the resizing, scrolling them, adding the metadata and other badges, interfacing to the database, saving metadata to files and to the database, updating previews and preview thumbs, handling the user interface, etc., now those are things that LR could do a lot better. The CR pipeline is already pretty good and largely not a holdup for most things.
It's possible to put more than simple pixel processing onto GPUs these days. That's where the term GPGPU came from, General Purpose GPU. That's why the supercomputers of today are really just massive numbers of GPUs configured in parallel, to hyperparallelize the hyperperallelism. It's possible to rewrite Lightroom to operate primarily off the GPU. You could solve all the performance problems. Most GPUs have at least a gig of memory these days, and even midrange ones have as much as three gigs. That much memory could be used to cache a lot of previews. There is a direct and ultra high speed pipeline between GPU memory and system memory, allowing massive amounts of information to be paged in on demand...and if that information is images, all the better, as it's optimized for that.
Processing a RAW...all of it, not just the demosaicing but the entire render pipeline, can easily be handled by pixel shaders. There is plenty of lag in Lightroom in the develop module when I run Lightroom full screen on my 30" CinemaDisplay. I have an extremely powerful system, an overclocked i7 4930K with 16Gb of high speed, low timing ram, and a pair of 4Gb 760's running in SLI. It's a massive amount of computing power. LR should be able to handle rendering a full-screen full detail image off a RAW at 30fps...it can barely handle 12fps (and that's with a D III 22.3mp RAW). A GPU would make it a no-brainer to achieve at least 30fps performance.
As I said before, it would probably take a rewrite of ACR. I don't doubt the current author that ACR, as it is currently written, couldn't benefit from a GPU. They would have to redesign it to take advantage of a GPU's parallelism. I don't think it's just a patch to do that...it would be a massive overhaul at the very least, if not a total rewrite. I still think it is not only valuable...it'll probably be necessary in the future if pixel counts keep increasing. General purpose CPUs aren't good at massively parallel processing. They have some parallelism, but it pales in comparison to what GPUs can do (especially when you use two or three or four of them together.)
And with that, I'm out.