The maximum throughput of the PS2 memory card slot is 24Mbit/s (3MB/s). In practice probably much lower. The DVD-ROM meanwhile is 4x DVD speed, or 5.28MB/s. Ethernet interface is 10MB/s and the HDD interface is 66MB/s. I'm not surprised that testers are having issues.
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The memory card bandwidth of PS2 isn’t great and not really made to load whole game data through it.