I'm having some issues sometimes with Material viewport preview size since updating.
When you switch from a lower size to a higher dimension it can freeze for 10-15 seconds or more and lags for a couple minutes before the viewport is useable again. It happens on Procedural and bitmapped materials.
Here's a boring video to watch of what i mean :P. I'll leave it to you to discuss. I can use 512x512 for now with about a second lag