For information sake, the DVD drive on the PS2 is capable of a maximum read rate of 2x. When reading CDs it's 24x.
It's CLV (Constant linear velocity) so the data rate is fixed regardless which area of the disc it's reading... Contrary to for example, the Dreamcast which were CAV (Constant angular velocity) meaning the disk spins all the time at the same speed but at the outer rim more data is read per second due to disc density per revolution variations.
You might think "what is wrong" with the USB loading then ? The data rate is more than enough... But then if you think that the PS2 has to play the game and drive the USB port you see there's not enough CPU time for doing both things. That's when games start to stutter. When you use the DVD drive to read a disc, the disc drive does all the work of fetching the data and then the CPU get the data chunk it needs in a nice memory buffer. No additional work necessary besides moving the chunk to the place it should be for execution.
With USB, the CPU has to control the data flow while the game is running so the theoretical limit in this situation is much, much lower than the maximum bandwidth speed...
The same limits apply if you think why the PS2 never had USB2.0. Remember, the IOP is only 36Mhz at the maximum speed setting and can drop to 33mhz when set up on PS1 mode.
There's simply not enough CPU to justify USB2.0 and even if it was USB2.0 the CPU isn't fast enough to drive it and get decent transfer speeds in real time. Since the real purpose of the USB ports on the PS2 were keyboard/mouse/USB networking, USB1.1 is enough.
On network, the networking hardware does some buffering and that relieves the CPU a lot from polling the accessed media. This alleviates A LOT the problem and is why the network module works so well.
A PS2 is not capable of using the full bandwidth of the network adapter... Anyone who used a XBOX1 as file server knows how fast a 100mbit ethernet link can go ...
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)