Mac Pro 5.1, Mojave 14.4
Graphics card: AMD Radeon RX Vega 56
Short description of error
By adding a simple object to a project (let's call it the 'critical object'), the DevLook viewport performance can be completely break down.
When switching off the visibility of the 'critical object' in the 'Scene Collection', the DevLook performance will be back to normal. Somehow, the combination of some objects or its properties are causing this problem.
Exact steps for others to reproduce the error
This can be verified by downloading the 'Book Factory.blend' example from here:
If you experience a very sluggish viewport performance in DevLook mode, just switch off the visibility of the 'Output Machine' object and the viewport performance will be back to normal. Switch it on again and the viewport performance will break down again.
I observed this behaviour also in other projects and by switching off and on the visibility of the different objects, one can figure out the 'critical object(s)' causing complete breakdown of the DevLook viewport performa.
Hope this is helpful for further investigation of the problem