STM32F407VG SDIO read and write speed
Count the number of written blocks per second.
You can probably hook into the SDIO interrupts or driver to get an integer increment somewhere. Or it might already have this, check the documentation.
Then in the systick for example, see every 1 second how many blocks were written. And reset the integer.

Or use the producing data thread, and increment an integer each byte you write to the file stream. Then see how big this number is every 1 second, and clear it.

For example, printf and related functions often return the number of written bytes.