slow flash drive write speed

[brudan]

I have a USB3-capable flash drive that holds a single 16GB fat32 partition. I need fat32 because this flash drive is used on OpenBSD, GNU/Linux, and also Windows (imposed at work). In the tests below I'm plugging this flash drive into the same USB2 port of my T400 ThinkPad.

When booted into my GNU/Linux partition, I can copy a 260 MB test file to this drive in 18 seconds.

When booted into my OpenBSD 6.4 partition, it takes 120 seconds to copy the same file to the drive.

I tried mounting the drive in OpenBSD with all of these commands, with similar results:

# mount /dev/sd1i /home/bruno/usb
# mount -o async /dev/sd1i /home/bruno/usb
# mount_msdos -u bruno -g bruno /dev/sd1i /home/bruno/usb

Surely OpenBSD must be capable of writing to a flash drive faster than 2.1 MB/sec, so the problem must be me. What am I doing wrong? I've been going around in circles and am not sure what else to try.

P.S. I also tried formatting the drive to ext3 (even though I can't keep it as ext3 for because of forced Windows at work) and got similar write speeds.