I don’t believe Unetbootin works for the Raspberry Pi. It’s a utility for creating bootable images intended for the desktop version, but doesn’t work the same way on a Raspberry Pi.
As far as I recall, the Raspberry Pi does need a SD card present to boot off a memory stick.
Do you many other USB drives plugged in at the same time? Do you use a powered USB hub?
It could be the case of too much bandwidth or insufficient power for the USB bus, so I/O operations are being held up when other devices compete for power and transmitting data between the device and Pi.
USB 2 devices and ports are naturally capped to a maximum of 480 Mbps (48MB/s). Most devices I’ve seen work up to 10 MB/s to 32 MB/s, depending on the age of the USB drive.
@Josele13 mentions about swap. When physical memory is low, data in RAM gets swapped to/from the disk, which would definitely take a hit to performance, even on SD cards (which is why it’s not used by default).
A tool like Disks or GParted will reveal if there is a swap partition. Try turning it off if there is one.