MacBook Pro 15" 2015 CPU throttles down to 0.8 GHz
Just to show that adding to a two year old question is worthwhile, imike's comment from only a month ago, was spot on for me (although waldgeist himself had already begun to suspect the same in his own comment from only a day after his original post.)

So I have this very same MacBook Pro model, and was seeing the very same issue. And that very cause -- an external HDMI-connected monitor -- is at play. 

The following shows two instances of it happening. In each case: something has caused the frequency to flatline; I remove the HDMI cable from the Mac; and within a few seconds things return to normal.



I still don't know how the external monitor (Samsung U32J59x) is having this effect, but it's a step forward to at least know it is.

ADDENDUM

Red Herring alert. Sadly, whatever this is, it turns out it is not specific to HDMI. I have now seen it with an old Apple Cinema Display. I've even seen it while driving an iPad as an extra display using Duet. In each case, detaching the external device solves the problem and the frequency returns to normal. But in fact, I've even seen it occasionally with no external display at all. For example, it will sometimes happen during a video call (with Zoom, at least). There, I can solve the problem by turning off the video channel in my call (i.e. using only the audio channel). 

So it may be as benwiggy suggested in a comment, something to do with graphics in general and the GPU. In other words, it's not an HDMI monitor, or even a monitor at all, but some function of an increase in graphics demand (which of course adding an external monitor would probably be an example). Push demand high enough and the CPU crawls into a corner and curls up; then remove the demand and the poor wee thing comes out of fibrillation and recovers. Sounds credible I guess.

One other point for anyone else chasing this thing. I can't be sure--maybe I'm now just fixated on it --but I could swear the problem is getting worse, week by week. If I do plug in an external display, it will happen, and fairly quickly. Five, maybe ten minutes at most, and then the frequency falls off a cliff and performance becomes barely usable. Then, detach the display and it will recover. It's less obvious when it happens when there is no display attached -- i.e. during a Zoom call -- but it's still highly likely. 

So, what aspect of the machine could be deteriorating week by week and so could create this getting-worse effect? Well, the battery for one. And I recall that a battery issue was mentioned in a related thread. Also, I did have a power issue with this machine, about a year ago. While debugging that, I did get a couple of warnings that my battery needed replacing. In the end the problem turned out to be a PSU issue, and getting a new one did seem to solve it. And so I put the battery warnings down to mere bad-PSU-induced delusion on the part of the Mac's battery monitoring system. But it's perfectly possible there was indeed a battery issue beginning to appear, in addition to the main PSU one.

So I'd say that may be a worthwhile lead to pursue.

But for me: well I just didn't have time to keep debugging and so I went for a much more certain solution. A nice, shiny, brand new 16" MacbookPro! :-)