I wonder how the write times change when they become over-write times, and sectors have to be erased before they can be written.
I'm surprised no one else responded to this; TFA didn't mention it either; I recall having experienced a significant performance difference between a new and a used USB stick in the past.
I happened to have an unused USB stick (SanDisk, 4 GB) lying around, so here are the test results I get right now. Out of the box, it takes about 4 seconds to write 40 MiB, using the command
time sh -c 'cp /tmp/random_data_40MiB /media/FBDD-DDE3/rnd40M-3; sync'
Strangely, sustained writes (looping the above command) are about a factor 2 slower; maybe there is some cooling-down time to prevent overheating of the flash chip? After filling up the entire USB stick and deleting all the files again, there is no difference in performance compared to when it was fresh: 10 MiB/s for a burst write and 5 MiB/s for sustained writes.