USB3 transfer speed is faster than USB2, USB2 transfer speed is faster than USB1. Connect USB2 flash drive to USB1 port in PC, transfer speed will drop to USB1 speed. And also from PC USB1 port to USB2 flash drive, maximum transfer speed also drop to USB1 speed.
Choose any one thumb drive without data, reformat to NTFS.
Save one AVI (DivX) File Size 1.GB，one MP4 (H.264) File size 1GB,one MKV File size 1GB to that thumb drive.
transfer to same PC USB Port , ( but don't transfer all together), mark down the transfer time, you will get the answer.
Transfer speed is only depend on the volume of video transfer, nothing to do with the type of video. The result come up should be more or less the same.
If still have the same problem, the only reason is either the USB port or USB drive somewhere faulty. USB drive faulty, can use Windows Tools defragmentation get rid of bad sector. USB port faulty can only not be use anymore.
Different brand and type and version thumb drives have different read and write speed, come up the result different is normal. PC better use NTFS than FAT32 or exFAT thumb drive to save data.