Low SD write speed with Arduino Nano 33 BLE and mbed os

# The setup

I am using the Arduino Nano 33 BLE with the nrf52 and it is running on mbed os.
The nrf52 is connected to an micro SD card adapter.

I have written a simple Arduino sketch with the standard SD.h library.
(I did a separate test with the sdfat.h library with the same result)
The sketch is writing 512 byte buffers to the SD card and logging the speed.
The used SD card is a class 10 SD card.

I have repeated the same setup with the same sketch with a different board the Arduino Nano 33 IoT which does not run on mbed os with the SAMD21 processor.

# The problem
When logging the speed I get drastically different results.

With the nrf52 and mbed os I am getting a write speed of approx 32-35 kBit/s speed.
While the SAMD21 is able to produce a speed of over 100 kBit/s.

So far I couldn't find the problem with the nrf52 and mbed os.
The arduino sketch is the same as well as the SPI clock.

I have also seen other micro-controller performances being much higher in the ballpark of 200 kBit/s as well.

So far I don't know where the problem exactly resides.
I can only assume it has to do with maybe mbed os.
But any help with that issue would be greatly appreciated since I need to reach write speeds of above 150kBit/s.

Start by reproducing the reported 512-byte SD write test on the Arduino Nano 33 BLE with mbed OS and compare it with the Nano 33 IoT using the same sketch and SPI clock. Compare the SD.h and SdFat results and measure where the write-speed difference appears; done means identifying a specific cause for the 32–35 kbit/s result or narrowing it to mbed OS, the SD path, or the hardware setup.

Written by the indexing model from the issue text.