Estimated copy time? Copying Finder folders of JPEGs from new iMac 10.11.5 to new SanDisk 256GB flash drive, USB 3.0.

129 GB is taking three hours. Seems slow to me, does this sound right?
It really depends on the write speed of your flash drive. But it does sound right since (like Sirens pointed out) it's many small files and not one big one.