Let me correct myself. I came across Jim Grey's 1999 paper "Rules of Thumb in Data Engineering" where he states that this heuristic is from Gene Amdahl in about

Let me correct myself. I came across Jim Grey's 1999 paper "Rules of Thumb in Data Engineering" where he states that this heuristic is from Gene Amdahl in about 1965: a balanced system has one bit of I/O per second for each instruction per second and one byte of memory for each instruction per second. So, 8 MIPS per MBpsIO, and one MB/MIPS.