Unless one uses industrial grade flash devices (very unlikely) then anything using low cost flash (e.g. USB sticks) can be extremely horrible with large scattered writes. And it gets worse if the file system is other than VFAT. Think seconds per write operation.
And I suspect the performance degradation is non-linear: hit it with too many writes and you will be punished accordingly (and then some). Any efficiencies up stream (e.g. huge pages) may just make things worse.