The article mentions that you can't use FPS as a yardstick for performance if you're already getting "max", but there is a way around this.
You can measure the frame start and end time (duration) and divide 1000/average duration over 1 second. This will give you the render speed even if that's not the actual number of rendered frames.
You're right, it's usually called the frame time and some engines even measure this per frame using GPU events/markers. However, the main reason for not using FPS is that it's not a linear scale. Losing 5 FPS when it's running at 120 FPS isn't a big deal but losing 5 when running at 10 FPS is a disaster. Comparing frame rates is useless, comparing frame times is the right way.
You can measure the frame start and end time (duration) and divide 1000/average duration over 1 second. This will give you the render speed even if that's not the actual number of rendered frames.