Are the partitions on a single disk? If so, you've just added the overhead of having to filter your data but you're not benefitting from having multiple disks to offset that cost. On a guess, I'd say that's it. Always remember, partitioning may, may, increase performance but it's primarily a data management tool so it frequently doesn't help performance at all. But that doesn't matter if you're able to manage data sets in a way you never could before.