@ESP_sprite, I did some further tests:

- using a class 10 SD card (instead of the class 4 I was speaking above) I get just about a 10-15% speed improvement

Following the observations of @gingerologist, the issue seems:
- not linked to the SD card (a class 10 is more than twice as fast as a class 4 ...

@ESP_Sprite, thanks for your reply.
In the example I posted, I'm writing chunks of 1000 bytes at each call of fwrite. I verified afterwards the output file
and it's written correctly.
I tried writing chunks of 1024, 4096, 65536 bytes as well and I obtained the same writing speed.

To give some more ...

Hello,

I just benchmarked the write speed to an SD card and found out that it caps at about 400kB/s.
I was surprised of such a low speed and read posts where users claim writing speeds of more than 1MB/s (up to 9MB/s)
while others have speeds comparable to mine but could not figure out the ...