9 ms is due to the checksums. Overall, for sequential reads, the time penalty of checksums is small. d. Assume that for each random read, you never have the checksum cached, and hence you must read it from disk too. Hence, for each random read, you must seek to the checksum (4 ms), read it from disk (negligible, just reading 20 bytes or more realistically a single block at 100 MB/sec), seek to the data block (4 ms), and read it too (again, negligible). Hence, the total time per read is 8 ms, and for all 10,000 reads, 80 seconds.
An Elementary Treatise on the Differential Calculus Founded on the Method of Rates or Fluxions by John Minot Rice, William Woolsey Johnson