USB slow when two devices connected

[Aldo Foot]

My F8 system has two USB ports in the front side.
I had a USB DVDRW connected to one of the ports and was writing an ISO
image to CDR.
Then I plugged in a USB stick to transfer some files from it to my
desktop, the transfer
was stalling and was slow even after the CDR task ended. Then I
unplugged the DVDRW
and the USB stick transfer rate was as fast as lightning.
Why was the USB stick file transfer so slow when the DVDRW was plugged in?

~af