VERY slow internal HD transfer performance

My MacBook Pro 17 (early 2011) came with Snow Leopard. Via some common-themed recommendations I found online, I repartitioned my drive, splitting the original single partition into two partitions, and 'freshly' installed a Lion on the 2nd partition. I then installed Apps, moved data, etc., and everything went off successfully. Lion runs well, and performance seems good.

I eliminated the original Snow Leopard partition and created a "Data" partition in its place (JHFS+) where I plan to move my media libraries to since they eat up most my Lion HD space (I can't just resize Lion since it's now the 2nd partition; the system won't let me re-capture the space at the beginning of drive and add it to the 2nd partition.)

When I copy between the two partions, the performance is horrible. I get, at BEST, 16.5MBps and that's if copying very large files. If I'm copying many smaller files, it's under 10MBps. I can copy to/from either internal partition to/from an external firewire drive and get 50MBps or better, so it's actually faster for me to do a two-step copy process; copy the Lion partition files to the external, then copy back to the internal Data partition. Then I at least net better than 25MBps.

Any ideas why my performance between two JHFS+ partitions on the same internal drive is so terrible?