I didn’t reply to your first post as I had no idea what was wrong. You now mention that you work with 500-700MB files, which would probably slow SketchUp down on most computers. I thought the 2018 version was a bit faster, not slower, so you shouldn’t be seeing worse performance on a newer version. It seems like you’ve changed both software and hardware at the same time, so it’s not so simple to compare the two.
Check your settings in System Preferences / Energy saver. Make sure it is the Radeon card that SketchUp is using and not the Intel integrated graphics, especially if you’re running off the battery.
Can you share an example file for us to test? Preferably a lot smaller than 500MB that runs slowly on your computer. Smaller than 100MB if you can, to save you uploading and us downloading huge files. Try and make the smallest file you can that runs slowly, to help isolate the problem. You can’t upload files bigger than 3MB here, so you’ll have to use something like Dropbox to share the file.