They allege (on the linked page) it could be as fast as 24 Mb/s, which while twice as fast as USB 1.1 (12 Mb/s), is still much slower than the theoretical maximum the PS2’s ATA-66 (66 MB/s) bus can handle (though that bus speed is likely more than even the best transfer speed of any DVD drive in a PS2 - solid state storage attached over ATA will likely remain the fastest choice, if it’s available to you)
In their early tests they have clocked 1.7 MB/s, which is faster, though only slightly, than USB’s theoretical rate (which translates to 1.5 MB/s)