f_sync() is generally pretty brutal and should be avoided. It is an equivalent to an f_close, as it flushes internal structures, fat tables, directory entries, etc.
The critical path to speed is to write large aligned blocks.
Doing small f_write() less than 512 bytes and spanning 512-byte sector boundaries being the particularly worst performers.
FatFs does not cache. Single sector IO command overhead predominates. FatFs maximum run length is a cluster.
Own the buffering, the file system and IO subsystem don't.
Best performance sweet spot, write 32 KB blocks, on 32 KB boundaries. Same for reads.
For course timing use Systick, measure across f_open, f_write(s), f_close
For fine timing on fast systems us DWT_CYCCNT.
I time read/write of 32 MB as 32 KB blocks. It will give you a performance ceiling for your implementation. Usually constrained by bus clock, bus width, and card.
Write speed on cards are often much lower than reads, the cards juggle 128KB erase blocks internally.
On premium cards I can probably sustain 15-20 MBps Writes and 25 MBps Reads, more realistically you might hit 6-7 MBps Writes. With dumb buffering you'll likely be below 1 MBps
