How old is the drive and how is it formatted? You’re gonna want ExFAT for the best performance with large files.
Try wiping the drive, creating a new GPT partition table and formatting with ExFAT. If that doesn’t improve performance, then it may just be the flash drive. I find that the drives that advertise their speeds tend to be faster than the drives that don’t, and I get the BEST performance from SSDs. I now have a pile of old flash drives that I no longer use because they are so slow.
Also, large flash drives (128GB, 256GB) tend to be a LOT faster than small ones. And as the drive nears it’s capacity, write speeds will start to slow down dramatically.