The PS2's CD/DVD drive is capable of about 5.4MB/s, in theory.
12Mbit is only 1.5MB/s. And that is the bandwidth of the USB 1.1 bus, and not the throughput.
The reason why we might occasionally come across a game or two that runs without stuttering is because unlike the CD/DVD drive, USB drives don't have such a large seek time.
Not only that, but some games are just efficient at reading data or they don't need such a high bandwidth.
Conversely, some games are just badly written and are just not very good at reading data efficiently. Such games will never run well from USB devices.
Lastly, 1.5MB/s is actually... very slow.