Measuring and Improving Serial Performance / Serial.flush() Blocks Forever on Nano 33 BLE Sense

Official Hardware Nano Family Nano 33 BLE Sense
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as stated, related but two different questions in my opinion.

question 1: why the hell is flush not working on the Nano 33 BLE Sense?
question 2: how fast can USB CDC be and how do you test / measure?

seems very different to me