YURII, as has been explained multiple times, 100% CPU is not the same thing, it all depends on which parts of the CPU software is using. Corona drives the CPU harder, or some parts of it, or makes the CPU require more consistent draw of power, than other software. It's as simple as that - the fact that some high level approximation says "100% CPU" does not mean you are comparing apples to apples.
Corona is known to use the CPU in a much more demanding way - ie taking full advantage of what the CPU can do - than other software, because we are so well optimized to use the CPU to its full advantage for rendering. Again, the evidence is in the fact that a 2 minute render on high power will run faster than a 2 minute render on low power mode, but after 5 minutes suddenly everything drops in speed - which can only be thermal throttling of some part of the motherboard or CPU. This then comes down to the design not being able to keep everything below throttling thresholds when the CPU is exploited so well.
This is not unique to Macs - if a PC had inappropriate cooling on it, it would do the same. If a motherboard had inappropriate cooling in any machine, it would do the same. It is impossible for software to ask a CPU to do more than it was designed to do - but it's entirely possible for a cooling system or motherboard design not to be able to support the CPU doing its maximum for an indefinite period.
So again, there is nothing we can do other than "cripple" Corona to NOT use the CPU to its best advantage - which you can already do yourself, with less threads, or low power mode.
I really don't know how else to explain it, so I guess if you choose to believe otherwise and have some explanation for why high power mode works at high speed for a period of time then slows down, then we will have to leave you to that belief.