The speed drops because at the beginning of writing the data will be
copied to the filesystem cache and only after this file system will try
to flash the cache to the usbdrive. If the drive is to slaw it will
reduce copy speed. Normally if file system mount with option sync  no
cache will be used and you will see real speed of your drive.

Do you have any linux configurations where it hase better speed ? What
say kernel log about this, are there any warnings ? Use dmesg to find
this out.

Try fallowing test. Umount your usb stick, start terminal and run this
commands:

sudo mount -o sync /dev/sdb1 /mnt/
# will mount with option sync usb stick if it located on /dev/sdb1 to the 
directory /mnt

sudo dd if=/dev/zero of=/mnt/test.dd bs=1M count=100
#will write 100M to usb drive and return writing speed.

My Kingstone usb-stick will return:
104857600 Bytes (105 MB) kopiert, 130,25 s, 805 kB/s
This is normal, real speed if sync used.
Write speed depends on filesystem block sice.

-- 
USB pendrive, write speed
https://bugs.launchpad.net/bugs/182931
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