I can see the difference clearly in my web browser.
You could try disabling the GPU to see if that makes a difference. (Preferences > Performance > Uncheck Use graphics processor) With a 5k screen you probably need the GPU, but you can at least try it for troubleshooting.
If it helps, you should update your graphics driver, but since you're on a Mac, I think you have wait for an OS update.
The difference between the two is clear as day to me on my calibrated 5K monitor.
When calibrating, make sure to create a version 2, matrix-based profile. Lightroom doesn't like version 4 and/or table-based profiles. I've never seen a bad monitor profile causing anything like this, but recalibrating is worth trying.
You can also try resetting the preferences, see https://www.lightroomqueen.com/standard-lightroom-troubleshooting-steps/
