I don’t understand why this thread is marked as “solved”, when it isn’t.
I found out the real reason with these write speed drops. This USB flash drive obviously uses some kind of cache, similarly as TLC SSDs use SLC cache. If you copy files that are smaller than cache size, you won’t notice any speed drop. But if you copy files that are larger than cache size, cache will fill up, and flash drive will continue writing to drive’s main slow NAND. You can verify that this is the case by cancelling of copying, and then immediatelly copying again - write speed will almost instantly fall down because drive hasn’t had the time to clear the cache yet, so the drives continues to write to main NAND instead of cache. If you cancel again, give the drive half a minute or a minute, and copy again, it will start at the full speed because now again it can write to cache, because it had enough time to clear it.
So, when you copy files to drive, it will first try to fill the cache. When the drive is idle, content from cache will be moved onto slow NAND.
I have 16 GB Ultra Flair drive - it seems that it has 384MB of cache (I suppose 3x128MB).
