With the new i9, it doesn't matter if it is plugged in. The cooling setup in the laptop is just inadequate to keep it running at speed for more than a couple minutes according to David Lee and other tech reviewers.

According to Lee, the i9 can't even idle without thermal throttling. I get having a chip with a higher max wattage than your cooling can support: that can still be pretty good for bursty workloads. But your cooling being so insufficient that the chip can't even idle? That's where the manufacturer is supposed to draw the line. I'm surprised that Apple even released this.

How do you show that it's "thermal throttling" at idle? Intel processors (at least, when managed under macOS) always downclock themselves when idle, regardless of how much headroom's available. You'll rarely (if ever) actually see the processor running at its quoted base frequency, even when plugged in.

Windows too. Just watching the task manager on any recent Intel can show you that the machine spends most of its time way under its rated clock, until you really start loading it up.

Yes, the cause for throttling is only thermal as long as your battery is over ~5% charged.

In fact, while charging performance might even be a bit lower because charging the battery generates some heat as well, further lowering performance.

Yes, it's mainly a heat issue.