100 mb only transfering only 10%efficent

i am wonderin if the is an overhead issue as to why i can only get about 10-12% effienecy out of a connection when both cards are 100mb and it is a x over cable between them i seem to get the same from my 1MB BB connecitons were my max download speed is 115k. just wondering as i would have thought that as this is a full duplex transfer between my 2 pc’s this should be faster

any info would be gratefuly accepted

joe