I’ve done that. I’ve processed 50 high ISO Nikon D850 images with Photolab 4, going back to check another 40 high ISO Nikon D810 images which I’d just processed the day before with Photolab 3 and switch them over to DeepPrime. The images are here, single sample below.
NIKON D850 - 200mm - 1/640 - f/2.8 - ISO 12800
Everything shot on a D850 was processed with Photolab 4 and DeepPrime. Everything shot on a D810 was processed with Photolab 3 but updated with DeepPrime for final export. There is some difference between colours in Photolab 3 and Photolab 4 with exactly the same adjustments. I had to fix a few images which looked wrong after the upgrade so be careful with reprocessing. It’s not just a case of reopening the old folder with the new Photolab: you’ll have to check your images individually to make sure the processing still looks right. It’s not wildly off but enough to change an image’s look for the worse.
Performance: my platform is a Mac Pro 5,1 with 12 x 3.1 GHz processor, 128 GB memory and an Radeon VII graphic card, all SSD with images on a separate SSD than OS. Monitors are 2 x 4K kept at HiDPI 2560x1400 + 1 x 2560 x 1200 kept at native resolution.
I had been having performance issues with Photolab 3, particularly with the contact sheet which was constantly reloading images and jumping focus around. Moving that contact sheet back from the 2560 x 1200 monitor at 72dpi to the HiDPI 2560x1400 monitor seemed to help. I also deleted the existing database and emptied the image cache at the same time.
I’ve always found sliders a bit slow in Photolab. Under Photolab 3, it was particularly important to avoid having NR enabled when trying to process an image. It seems much less important under Photolab 4. Preview times are almost instantaneous with very short wait times to see the changes. From the outside, it looks like the image preview caching has been completely rewritten to work more like C1 (faster previews but sometimes less exact renders: far preferable to an impossibly slow preview time).
Photolab 4 general performance while editing on this heavy duty hardware (an Radeon VII with 16 GB VRAM and 128 GB memory is a best case scenario) seems much more spritely than Photolab 3. I would be glad to have upgraded just for the performance improvement. I hope that answers your question.
PS. That the performance improvements are so considerable, it makes it all the more frustrating that DxO has decided to cut off all 2011 MBP (last 17", last anti-glare, come with i7 quad core processors) from upgrading to Photolab 4.The last OS 2011 MBP can run is 10.13 High Sierra which should be within Photolab 4 requirements (last three Mac OS). I’ve paid DxO again and again and again to get performance improvements. When they finally run, I cannot benefit from them on any of my laptops. It makes me feel that I’ve been misled and that DxO is taking advantage of their loyal users. It’s a pity to be forced into such mixed feelings about what otherwise would have been an excellent update.