Brush mask tool is so laggy that it is completely unusable

[cltexan]

Like the title says, I'm unable to use the brush tool at all for masking. It is so laggy it freees my screen when i try to possition it over an image. This has been a problem for about a year, but I've ignored it and used Photoshop when a specific need arises. To be clear, the tool is set up correctly... Flow and density at 100% and I'm definitely on Brush (not Erase). 

I'm using Lightroom Classic 11.4.1 on a 2017 iMac 5k Retina, 64 GB with the stock 

Radeon Pro 570 4 GB 

Any help would be ppreciated. Thx!

[AxelMatt]

The first thing that you should try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.

https://helpx.adobe.com/lightroom-classic/kb/lightroom-gpu-faq.html#troubleshooting

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html

Another step is to try to reset the Lightroom preferences.

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

It's recommended to backup your preferences before you reset the preferences to the default settings:

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Have you tried to install the latest update to Lightroom 11.5?

[dj_paige]

With a 5K screen, your GPU should be ON (Preferences->Performance->Use Graphic Processor).

With brushing, you should try it with Auto Mask off.

With a 2017 computer, it may not be powerful enough to drive a 5K screen local adjustments.

[GoldingD]

That 5K screen, external? Can it be used at less than 5K?