Since you have a slim i recommend loading via network. It's noticable faster than loading from usb, but still slower that disc or HDD.
I think the speeds are about the following:
usb: theoretical max speed 1.1 MB/s
LAN: should actually be 1-2 MB/s
disc: up to 4x dvd = ~ 5.5 MB/s(the more the data is away from the middle of the disc, the closer it gets to that speed)
HDD: depending on the HDD, but should always be faster than disc
As loader i recommend Open PS2 Loader, it does USB, LAN and HDD loading, and is still in development, unlike USB Advance and HD Loader.