P: Slow UI when using Mac and Custom Display Profile

Apple says all IMac Pros are metal compatible

This is a good example of when Adobe would really appreciate a video recording of what you're seeing. Regarding the font, it seems like it was intentionally changed throughout the interface. Win 10 here.

Glad to read this as I noticed this slowdown on my iMac too. (I reverted to 9.2.1 though, due to the clone/heal bugs when used at the frame edges, which were introduced in 9.3).

Upon upgrading from v9.4 to v10, certain UI actions in Lightroom have become incredibly slow.  The most noticeable is scrolling in Library Grid view.  I experienced no lag/hesitation in any of the 9.x versions, but in version 10, scrolling in Library Grid view is almost unusable (several seconds to scroll a couple of rows).  I downgraded back to v9.4 and all lag went away.  I'm running a 2016 Macbook Pro 16GB RAM with Radeon Pro 455 2GB VRAM. MacOS Catalina 10.15.7.

In the Performance tab in Lightroom's settings, it says the the GPU is fully supported. Plus, all the actual heavy lifting editing functions work quickly as expected. And actually, now that the adjustment brush is GPU accelerated, that works like a dream as well.

It really just the UI that took a hit this update. At least on a Mac, I can see something definitely changed in the way the UI is rendered but I can't put my finger on it. The font in the Develop module's drop down lists is different. Something was definitely updated but there's clearly a bug here.

I appears that your video card is unsupported. Technological progress I guess. 

On Windows with both 4GB and now 5GB Nvidia Quadro video cards (4K monitor), scrolling was fine but has slowed down with V10 but only a little; just enough to notice. 

@bill_3305731 6GB of VRAM isn't enough to scroll some content in a window? Come on… Especially since it worked nicely in the previous version and every other application - including other Adobe ones.

The Vega 56 is on Apple's maybe list. "Metal" support is required for Lightroom to make use of the card. 

The following cards are known to be Metal-compatible: 

MSI Gaming Radeon RX 560 128-bit 4GB GDRR5
SAPPHIRE Radeon PULSE RX 580 8GB GDDR5
SAPPHIRE Radeon HD 7950 Mac Edition
NVIDIA Quadro K5000 for Mac
NVIDIA GeForce GTX 680 Mac Edition

 
Apple also lists cards that "might also be compatible" with macOS Mojave: 

AMD Radeon RX 560
AMD Radeon RX 570
AMD Radeon RX 580
AMD Radeon Pro WX 7100
AMD Radeon RX Vega 56
AMD Radeon RX Vega 64
AMD Radeon Pro WX 9100
AMD Radeon Frontier Edition

Here is my card - The Radeon Pro Vega 56 is a professional mobile graphics chip by AMD, launched in August 2017. ... AMD has paired 8 GB HBM2 memory with the Radeon Pro Vega 56, which are connected using a 2048-bit memory interface.

Tip, on Windows, Lightroom gives the best performance in single screen mode with the Lightroom window at full screen. 

Sad to say but you folks are using old and slow video cards with old and slow processors. Also as ATI is primarily focused on the gaming market, $for$, their cards tend to be slow for photo or video editing. 

Yes, Nvidia needs some competition in this area but since the effective demise of Matrox; they are the only game in town. Maybe we could crowd fund a video card focused on photo editing, just joking of course as we'd still have to use a Nvidia chip. 

Recommendation for 4K monitors, consider a 8GB Nvidia Quadro if there are drivers for macOS. For 5K monitors, 11GB would be better. 

Lastly, each new release of Lightroom provides several performance improvements and usually at least one slowdown. For V10, we took at hit with scrolling in, mostly, the Develop module. But the balance overall is a significant improvement. On my machine (6-core Xeon, Nvidia Quadro, dual 4K monitors), the hit is very minor.