I have a late 2017 iMac with an i7 4.2GHZ, top of the line graphic card and 40 GB of RAM, and the fans will kick at full speed every time I'm working in LR, even with 1:1 previews and smart previews built beforehand. Strangely, that issue disappeared with LR 7.2 to return in 7.3. That doesn't happen with any other software. I've read in other threads that it might be an i7 Kaby Lake issue with LR, but if the problem was solved in version 7.2, what happened for the issue to come back again with version 7.3? One thing is for sure. Lr 7.3 (and 7.3.1) is much slower especially in Develop mode and the fans are kicking at full speed. I have Sync, Face Detection and Address Lookup always paused, so no issue there. For example, I just tried to move the exposure slider and the CPU percentage went up to 605% (see attached screenshot) and the fans just fired up at full speed, even with GPU acceleration turned on and off. That's what I think it's not normal and it was an issue that I've only had in version 7.0, 7.1 and now in 7.3. Version 7.2 was the only one that seemed to fix the issues, the one where Intel helped Adobe.
I understand that Lr needs a lot of processing power and that produces more heat because of all the transistors of the CPU and GPU, especially on the retina iMac and MacBook Pro and that Adobe spent the last years trying to make it faster and addressing all the user concerns. But if the latest Apple computers use even more powerful graphics cards, more powerful processors and faster RAM, shouldn't Lr be faster and use all that power to run smoothly? How come a lower spec 2015 retina iMac runs faster and without issues than a late 2017 top of the line iMac with 40GB of faster RAM? And how did all those issues disappear in version 7.2 to reappear in version 7.3? Editing was a breeze and now it's a real pain. C'mon Adobe, figure this out ASAP, please.