The principal advantage of higher spindle speeds
is not higher transfer rates but instead lower rotational latencies
and thus reduced access times
. An increase from 10k RPM to 15k RPM operation yields a reduction in average rotational latency from 3.0 milliseconds to 2.0 milliseconds. This 1 ms reduction is more significant than it appears... after all, it's applicable in -all- seek situations, whether it be a 20 millisecond full-stroke or a sub-one millisecond track
According to WinBench 99, the Ultrastar 36Z15's measured access time clocks in at 6.7 milliseconds. When taking the 2.0 ms of average rotational latency into account (access time = seek time + latency), the drive's measured seek time weighs in at 4.7 milliseconds, a bit off of its claimed 4.2 ms. Even so, such a figure is low enough to undercut the Seagate Cheetah X15 by a hair... a margin of just 0.1 ms.
The 36Z15's increased areal density allows it to sustain transfers higher than that of the X15, though not by as large a margin as many may expect. The drive's score of 49.1 MB/sec bests the X15 by about 20%. A current-generation 10k RPM drive like Seagate's Cheetah 73LP, however, outraces the 36Z15 by a significant margin.