Sorry that you're facing Brush and Patch tool lag in Photoshop.
Are you using the latest version of Photoshop 2021 v22.1.1?
Are you using a tablet to draw in Photoshop? If so then, which tablet are you using and have you checked that tablet drivers are updated to latest version? If you're not using a tablet, then do you notice the lag while working with third party or custom brushes?
Couple of troubleshooting steps if you're using a tablet, that you can try to resolve the lag issue:
You may also restore Photoshop preferences to default, Please refer this article for instructions on how to restore Photoshop preferences:
Let us know how it goes.
You have a bunch of files open and are working on a 16bit image. Can you work on 8bit images? I think you are taxing your system - how much RAM do you have?
I guess I didn't notice that I was working on a 16bit image. I'm assuming I don't need to do that? 8 bit will be fine?
I have 64g RAM
Try 8bit and see if performance improves.
Where do I change that setting? I can't seem to find it.
I think it's a graphics card issue. First, try resetting your preferences. If that doesn't work, you can trouble shoot, https://helpx.adobe.com/photoshop/kb/troubleshoot-gpu-graphics-card.html