Been using Edge for the first month after installing W10 and yes, I also notice that "slow down " thing. My next option was to use chrome. You can try it too.
Hmm, I have 2 flash drives but haven't found any write protection on it, if yours have one then Frosty is correct if not, try uninstalling the driver of your usb ports and reinstall it back so you can try using it again.