I have always had performance problems with sketchup at times across several versions 2016, 2017 and 2018. It will be working ok (it could never be accused of being a speed demon) but then it will start slowing down. It usually starts when I try to rotate my drawing. It will rotate very, very slowly taking several seconds to finally stop. Once I counted up ten … 1001, 1002, etc before it stopped. I have 2018 Pro on my Macbook and now it is really slow. Select something and it is three seconds before it is selected. Delete another 5 seconds. The wierder thing is that it does not seem to correlate to anything I’m doing or running.
I have tried lots of things found on the net. Nada! Still slow.