P: Slow UI when using Mac and Custom Display Profile

I'm pretty sure it is with the construction of the LUT's - since simply changing from one to another on the same 2nd monitor makes a difference ... but why is it a factor at all - what got changed in v10 that makes it dependent on this .... the color grading implementation infrastructure, perhaps ?

Having the same issue. The v10 update has slowed down my editing on a 2019 MacBook Pro with 32 GB RAM. I'm feeling that Adobe should pay more attention to performance and less to some fancy features. 

I run a 5.1 MacPro with no Thunderbolt. Thunderbolt is not the problem.

I'm another person with a 2017 MacBook Pro who upgraded to V10 only to find the UI slow and the Adjustment brush so slow as to be unusable. 

With a course to teach tomorrow I had to abandon V10 and went back to V9.4 where everything worked perfectly.  I'm running 10.15.6 but as others have said, this release runs perfectly and the path back to 9.4 was painless!

The only other item that this thread makes me think of is that I have two monitors (laptop screen and a BENQ Sw271) although they are used in duplicate mode rather than in extend mode (problems with Zoom otherwise!). Also that both monitors are custom calibrated with an X-Rite I!Pro.

Finally, could Thunderbolt be a common thread here with V10 (external drives, external monitors etc).

I am finding the same thing on an almost identical Mac (MacPro 'trashcan' Late 2013, 6-core, dual D500's, 64gb).  I have 1 DELL 32" 4K monitor (that's my main one) and a DELL 3011 30" - both running on display port.  macOS MOJAVE same as above.

With the 2nd monitor NOT running in LR - performance is fine.  Turning on the 2nd monitor and leaving the grid on the main display and the fullscreen image on the 3011 results in a 10x slowdown (e.g. from virtually instantaneous images switches on the 4K grid to now 2-3 seconds).  Fairly similar if I reduce the window on the 3011 to less than fullscreen and 'window' it.

But if I move the window from the 3011 over to the 4K DELL and run them side-by-side, then I am back to normal performance.  So using the 2nd monitor is a problem.  

For the 2nd monitor, the DELL 3011, I am using a display Profile I built in July using my ColorMunki.....

When I switched that display's profile to generic sRGB IEC61966-2.1 then all performance returned to very acceptance levels .... e.g. virtually instantaneous switch between images in full screen on the 2nd monitor.  Good solution ... sadly, the color is not right on the 2nd monitor.

So, not sure where to go with this ....

I just profiled my monitor using XRite colormunki and it worked. v10 is now working as it should. Just to double check I switched back to a different profile and it went back to being slow.

There is a relationship between the size of the previews and the speed of switching in loupe view. On my machine (Windows 10, 4K monitors), the following gives essentially instantaneous switching:

• Standard Preview Size: Auto (3840px)
• Preview Quality: High
• Automatically Discard 1:1 Previews: Never

If the Loupe size is set to other than 100%, there is a slowdown but it is very slight; perhaps 1/4 second at most. 

My full hardware configuration: 

HP Z440 
Xeon E5-1650 V4 6-core 3.6/4.0 GHz
64GB ECC RAM
Windows 10 Pro 64-bit 
Nvidia Quadro P2200 5GB (default scaling) 
Dell 27" 4K P2715Q 
Benq 32" 4K PD3200Q 
Samsung 950 Pro 512GB - C-drive
Samsung 860 EVO 2TB 
   -dedicated to Lightroom 
   -Samsung Rapid Mode active 
   -on a PCI-E adapter 

Yes, I find that switching to a default monitor profile (e.g. sRGB IEC61966-2.1 or ) then restarting Lightroom does speed up switching between photos with the Loupe.

However when using my custom monitor profile, I also noticed that the slowdown only happens when the loupe is less than 100% (with switching between photos taking 3-5 seconds each). When the loupe is 100% or more, switching is very quick.

Seeing as v10 added a new feature letting the loupe zoom to any percentage, it makes sense that this may be where the bug is.

I can also confirm that switching my monitor color profile to sRGB made it much faster. Pretty much any other color profile made it slow, including my custom calibrated profiles.
Sent my findings to Adobe (a couple of their engineers emailed me)

In my case, it was a fresh installation with the standard iMac profile. So, it is not just the profile IMO.