Testing with SanDisk 32GB SDHC HighSpeedClass1 (80 M/s rating)

debian@beaglebone:~$ cat /sys/block/mmcblk0/device/preferred_erase_size
4194304

FAT32 SD Card Partition

debian@beaglebone:~$ sudo flashbench --open-au --open-au-nr=5 --random --blocksize=4096 --erasesize=4194304 /dev/mmcblk0p1
4MiB    19.7M/s
2MiB    18.6M/s
1MiB    18.4M/s
512KiB  15.7M/s
256KiB  13.6M/s
128KiB  15.1M/s
64KiB   12.9M/s
32KiB   10.5M/s
16KiB   7.5M/s  
8KiB    5.12M/s
4KiB    2.98M/s

ext4: SD Card Partition

sudo flashbench --open-au --open-au-nr=5 --random --blocksize=4096 --erasesize=4194304 /dev/mmcblk0p2
[sudo] password for debian:

4MiB    19.4M/s
2MiB    19.1M/s
1MiB    14.4M/s
512KiB  17.6M/s
256KiB  15.8M/s
128KiB  15.1M/s
64KiB   12.6M/s
32KiB   9.93M/s
16KiB   7.42M/s
8KiB    5.03M/s
4KiB    2.94M/s

FAT vs ext4 Formatting makes no difference.