Generally, all processing of RAFs is slower than other raw files which use Bayer patterns.
I don't think so John. But maybe....
I've been a prolific shooter of Fuji for ten years and probably have processed more RAF files in LR than any other human being on Earth, and I have hung out on Fuji Boards for a decade and never heard that said. I guess it is possible for Fuji APSC XTrans files to build previews slower, but I don't think so.
I shoot medium format GFX RAF files now and those are Bayer. But what you say is interesting and I'm not ruling it out because I know that for many years Adobe had a problem handling the demosaicing of Xtrans RAF files in ways that caused much angst in the Fuji community, and they were attacked on the issue almost daily by the marketing trolls from Sony and Nikon on the camera forums (especially DPR). There was some watercolor and paintbrush effects in foliage and brick, for example. but that problem was overcome 3 years ago.
Anway, I have imported hundreds of thousands of RAF files, both X and GFX RAF, and always build 1:1 previews on import 100% of the time. It takes various amounts of time on the various computers I have and it is a CPU and memory intensive operation that does not use the GPU (unfortunately).
While previews are building, it maxes out the CPU, memory, cores and threads and you are out of action and aren't doing anything in LR till it's finished. I don't notice a difference in preview building speed between my Fuji X (APSC XTrans) and GFX (Medium Format Bayer) RAF filles and my Canon CR2 or my Leica 50 MP DNG files.
So, if that's the reason for his slower preview building with RAF files, you have taught me something I didn't know. But I don't believe it.
Do this - Someone go onto the Fuji DPR Board this morning and ask that question. I can't do it right now because I'm on a little vacation on that Board because I got into a friendly spat with a DPR staff guy named Rishi (the chief of their science team at DPR) who is a good guy, but we had a little minor disagreement on a matter concerning Nikon and Sony vs how good Medium Format GFX files are.... Never mind. I'm on a little DPR sabbatical right now. LOL.
But there are a couple of digital scientists on that Board who will answer. They know and have taught me a lot about Adobe and RAF files.
Just tell them Greg 7579 wants to know. The Mods will answer as will the scientists.
Now you have my curiosity up.
But I bet there is no way an XTrans files builds a slower preview than other proprietary raw formats. But I admit it is possible, because how LR handles XTrans files has always been a mystery, and Adobe is very quiet about it.
Stop struggling with Lightroom! There's no need to spend hours hunting for the answers to your Lightroom Classic questions. All the information you need is in 