Please help: one partition MUCH slower than the others

Hello all, For some reason, one of my partitions is MUCH slower than the others. Any other partition, even on the same disk, gives me speeds of 40-50 MB/s. This one however gives only 4-8 MB/s. This is independent of the partition being mounted or not. smartctl does not give me any errors, and fschk didn't find any problems either. Any thoughts/suggestions? Regards, Pieter Hulshoff