I get between 15 to 20 mb/s. I have an external hard drive enclosure which I use to load my PS2 drive with from my PC. The enclosure has both USB 2.0 and firewire ports. I've only used the USB port.
For those experiencing slow speeds, perhaps you have an antivirus program running that checks the iso images while transferring the file from your PC to the PS2. Try turning off your antivirus.
PS2 NTSC US V1 | PS2 Network Adapter | Maxtor MaXLine Plus II 250gb HD | Matrix Infinity v1.38
PS3 Slim | WD Scorpio Blue 750GB Internal | WD Essential USB 3.0 2TB