The transfer rate is a bit if a red herring, frankly, although you are right that the USB interface might not be as fast as the attached disk.
USB 2.0 allows up to 480mbps, and real-world tests suggest that 320Mbps should be possible. That's 40MBps, which is actually higher than the hard disk - IDE Ultra DMA, ATA-33 throughput.
OK, if you have ATA 66 or 100, then the disk will be 2 or 3 times faster than the USB interface it's plugged into (but that is not an order of magnitude different - unlike the difference between RAM and disk transfer speeds).
Even then, the scenery files will only need to be loaded from disk into memory once (and you won't be loading all 20GB at once!).
If there is insufficient physical RAM, then the paging file will be used - and that's on the existing hard disk.
So all in all, there's a negligible effect of using an external disk.
Of course, if the lappie only has USB 1, then that's a completely different story! 12mbps
SD