SD card vfat partition 12x slower than ext4. Any ideas?

I've upgraded a Yun and successfully used the sketch to setup an SD card (class 4 8GB Kingston), and there's a crazy difference in performance between the ext4 main partition and the vfat data one. Creating a 1MB file using:

time dd if=/dev/zero of=zero count=1024 bs=1024

takes around 10 seconds for ext4 and 2 minutes for the vfat data partition.

Any ideas why the disastrous fat32 performance?

The mounts are:

root@pingpong:/mnt/sda1# cat /etc/mtab
rootfs / rootfs rw 0 0
/dev/root /rom squashfs ro,relatime 0 0
proc /proc proc rw,noatime 0 0
sysfs /sys sysfs rw,noatime 0 0
tmpfs /tmp tmpfs rw,nosuid,nodev,noatime,size=30560k 0 0
tmpfs /dev tmpfs rw,noatime,size=512k,mode=755 0 0
devpts /dev/pts devpts rw,noatime,mode=600 0 0
/dev/sda2 /overlay ext4 rw,sync,noatime,nodiratime,user_xattr,barrier=1,data=ordered 0 0
overlayfs:/overlay / overlayfs rw,relatime,lowerdir=/,upperdir=/overlay 0 0
debugfs /sys/kernel/debug debugfs rw,relatime 0 0
/dev/sda1 /mnt/sda1 vfat rw,sync,relatime,fmask=0022,dmask=0022,codepage=cp437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro 0 0
none /proc/bus/usb usbfs rw,relatime 0 0

Just to be sure, having reformatted the sda1 partition from the default vfat to ext4, the time to dd 1MB dropped from 120 seconds to around 8 seconds, slightly faster than the overlay partition.

Don't know for sure, but the ext4 is mounted with "noatime,nodiratime" that skips writes about file and dirs access time.