System Information
Operating system: Windows 10
Graphics card: GeForce 1080

Blender Version
Broken: 2.80 690478027bd7

Worked: (optional)
2.79b with Window Draw Method set to Overlap Flip

Short description of error

Texture painting in the 3D viewport is laggy in 2.8, especially with a medium to large brush size. The smaller the brush size, the better the performance. Painting in the UV viewport has no lag.

In 2.79, changing the Window Draw Method to Overlap Flip fixed this problem. I have been unable to find the Window Draw Method settings in the 2.8 prefs.

You can test this on the attached object. Just create a new image and try painting on it in the 3D viewport. With a GeForce 1080 on Windows 10 with 32GB of RAM and a 3.6GHZ Xenon processor, I get extremely laggy performance.