When a "Baking Tex" node is in the scene, refreshing the Preview render is interrupted by "Upgrading Shader" pauses whenever something about the material is changed. Even if the change has nothing to do with the "Baking Tex" branch - even if the "Baking Tex" is not even connected.
Here's a screenshot of a setup that slows the Preview down.
In this case the shaders are updated, whenever I change something no matter where in the material (Diffuse nodes branch for example). The "Baking Tex" node isn't even connected to the Displacement node.