Yes that is common to my stress tests and benchmark tests so far... I haven't done the camera work in the middle but you can see that effect if you run the 20 cycles of stress testing... after 3 or 4 loops the CPU settles into a position where it can sustain the performance rather than short bursts. After the stress tests have completed and some cooling time, then this improves again. However if you watch the stress testing during it running, it is not particularly apparent to the 'user experience' that the performance is being reduced to be able to sustain a decent performance.
I think you would find that with any CPU... even in a PC or laptop used for high demanding tasks. I use a gaming laptop and throttlestop... the Intel CPUs use a 'Short Turbo Power Limit' and after a set period (say 1 minute), it switches to a 'Long Turbo Power limit' which can be sustained for longer.
In terms of 3D mark benchmarking, these routines are designed to be high intensive on the CPU load to push it to the max... but don't often show a real life 'use case'.
I am always cautious to make too many assumptions on benchmarks, either by phone, or by gaming PC optimising... it is the real user experience that matters most, and often gives some recovery time to the processor during loading screens, periods of less intensity.
My screenshot below shows the 3D mark stress test results across 20 consecutive runs. You can see it reduces after a few runs... but then sustains the level of CPU performance for the rest of the test.
Kind Regards
Smiley 
