Okay, so it turns out that the temp drive d: is a speed-demon.
MS warns that it can be cleaned out at any time, but it looks like it should be used for temporary paging, rather than storing ISOs.
Check out the scores from Roadkil:
c: 41.84ms 9.33MB 247.65MB
d: 0.03ms 2.97GB 1.18GB
z: 25.27ms 33.34Mb 10.05MB
Download Roadkil disk speed from http://www.roadkil.net/program.php/P13/Disk%20Speed