The RamFAST SCSI card in my Apple IIgs can push over 500kb/sec with DMA enabled and an old Apple 40SC connected to it (the card also has 256k cache on it). Theoretical max transfer speed on that machine is 1MB/sec (8bit bus running at 1Mhz). Somehow I think a machine powered by a 32bit CPU/bus could do better even if it was an older SCSI chip. It could also be the Ethernet controller requiring a bit of CPU time to handle packets combined with other delays.