You shouldn't expect performance to increase linearly with the speed of the processor. Generally, it doesn't. You should also consider other factors. The bottleneck may be elsewhere, such as disk and/or RAM. There may be more load on one machine. Etc. My gut feeling is that a drop from 6 seconds to 3.5 seconds is actually pretty good given the differences you cite.

-sauoq
"My two cents aren't worth a dime.";