The reason for this is because the IO capabilities. On a regular computer you can have your disk writing and reading at maximum of 100-300mbps even more on high end hard drives. Internal memory acting as drives on small devices are limited to about 4-5mbps depending on the bus speed to the board, and this varies as applications are also reading and writing.

So while your computer at 54Mpbs connection can accommodate 54Mbps write on a 100+Mbps ATA, a small device can only get what it can write to its memory. It is hardware limited and the same goes for any other device. I might also note that the Dell Streak's IO has the best score in benchmarks over any other phone, tho it lacks in the graphic speed department, and I mean that the FPS are limited to 30 while others go 60+.