It is always wise to try to find the read /write speed data before buying. That is said as I've had the same type of problem myself, it isn't intended as a criticism.
I have so called USB 3.0 flash drives that have write speeds that are slower then standard USB 2.0 drives.
A Kingston USB 2.0 drive drive gives read'write speeds of 18MB/s - 10.5MB/s
A USB 3.0 16Gb Kingston Data Traveler gives read write/speeds of 132MB/s and 8.4MB/s
On the other hand CrystalDisk reports my Sandisk Extreme 64Gb as having read /write speeds of 200MB/s and 180Mb/s respectively.