HDD > SMB > USB
So running retail games off the HDD is the best method, period. Well, the slim doesn't have a HDD bay so steaming over the LAN via ethernet (SMB) is the next best for that model. The USB is 1.0 speeds so expect shuttering on all models.
Well, check this out:
using a Raspberry Pi 3 + Open Media Vault + isos on USB (plugged into pi) + OPL SMB function
Essentially, streaming games directly off a Rpi3 (no router) to your PS2 via ethernet cable. While it isn't HDD loading, I think this awesome for the preservation of PS2 slims & a nice "in between" set up. I love this mix of new technology with our hobby to ensure it lives on, in this case after the disc drives begin to fail.
Videos by Project Phoenix Media (not me):
Proof of Concept:
https://youtu.be/V4kwRR8PEeQ
How to:
https://youtu.be/e0Pzp5Q3Ov4

Now can we say?
HDD > SMB via RPi3 > SMB over LAN > USB 1.0
Thought about posting this in news but because it is a new take on the existing OPL line up of methods, decided to land here.
While the original stock setups will always have a nostalgia spot for me, it is a VERY AWESOME time for our hobby between emulation, clones, flash carts, ODEs, HDD loading, and all the new mods out there.