I don't have access to another computer. But my computer (laptop) has one 4-pin Firewire port with a external firewire-only harddrive connected to it. I was originally going to daisy chain the harddrive and Plextor but Plextors manual warned against doing so. So I use the 716UF on the USB port.
Today I disconnected to harddrive and connected the 716UF to the firewire port. I used the exact same movie DVD (a 5.95GB movie) that I did yesterday to test the 716UF on USB. Oddly it worked fine. Started ~5.0x, dropped to ~4.0x, then gradually got to ~10.5x (+/- .3x) by halfway through the rip then gradually reduced spead during the last half of the rip.
Next I disconnected from the firewire port and connected back up to the USB port. Tested the same DVD and suprisingly got the same (~10.5x max) result I got on the firewire port. Have no idea what changed or got reset but I'm glad it did.
I swear, I tried this same DVD yesterday on USB four times with the same poor ~3x ripping speed. Thanks for the suggestion G@M3FR3@K, and everyone else.