See my in-depth review of the 2018 MacBook Pro:
MPG did a 500 iteration test which is GPU intensive. The scatter plot below shows how performance declines somewhat within a minute or so, then holds steady at the reduced rate. It turns out that the decline is far less than with the 2016 MacBook Pro or 2017 MacBook pro (see
Kudos to Apple for delivering a desktop-class machine—the 2018 MacBook Pro is a winner.