Operating system: Windows 7 Pro X64
Graphics card: Nvidia Geforce GTX 980 Ti
Broken: Date: 2019-01-21
Hash: dd3f5186260e Branch: Blender 2.7 (although it's supposed to be a 2.80 Beta)
Short description of error
Subdivision surfaces are fast and responsive in Edit mode with all-quad meshes, but very slow (~ 10x slower) with meshes comprised of triangles and quads.
Exact steps for others to reproduce the error
Open the attached blend file and select Cube.001
Go into Edit mode. Turn on Proportional Editing. Grab a point and move it - observe how responsive the viewport is with the all-quad mesh.
Go back to Object Mode. Select Cube. Go back into Edit Mode. Grab a point and move it (with proportional editing still turned on) Observe how slow it is.