I installed the Intel Power Gadget to check if anything can be shown.
Then one time I left YouTube running, showing 1080p, and placed the Macbook 12 inch on the bed, and an hour later, I saw that the notification said CPU performance has been reduced, and Intel Power Gadget showed:
so it seems like the Blue Line (for the processor) is showing that it was running at 1GHz.
And then I stopped YouTube, and put something under the Macbook so that it can ventilate better under the notebook base. After 30 minutes, without rebooting the machine, I ran YouTube again and some other apps, and the Intel Power Gadget showed the Mac running at 2GHz or above again. So it looks like the processor speed got bumped up back to normal, but it won't show you any notification:
P.S. When I have some apps running and Google Chrome running with a dozen tabs, the temperature kept on being 60°C and it won't jump back up to the 2GHz range. I had to quit the apps and Chrome and the temperature went down to 50°C, and after about 30 minutes the Macbook went back up to 2GHz.