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Cycles material changes on high poly meshes cause UI lag
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System Information
Operating system: Windows 10
Graphics card: GTX1080Ti

Blender Version
Broken: 2.8 latest master

Short description of error
When rendering with Cycles, material changes lag the entire user interface depending on the polycount of the objects given material is applied on. The lag is not so long to indicate the entire geometry is getting reparsed, but it still doesn't make much sense that polycount has any influence on the UI update rate. It makes working interactively with high poly scenes very difficult (for example shading high poly car models). Here's a short video of the issue:

Exact steps for others to reproduce the error
1, Switch renderer to Cycles and enable Cycles viewport render
2, Create a simple geometry and create a new material for it
3, Tweak parameters of the new material and notice the UI does not lag
4, Add two subdivide modifiers on top of the geometry to increase polycount to at least 2 000 000
5, Tweak the parameters of the material and notice the UI lags heavily