That translates to about 7.4MB/s. Indeed not very impressive but then again that card is only labeled as class 6 so anything above 6MB/s is a bonus.
Try formatting it with ext2! That should bring some performance gain compared to fat (maybe an MB/s or two if you're lucky).
But realistically we won't see any exceptional numbers with that card anyway.
I'd estimate the limitation on the N900 side at about 30MB/s, so buying SDXC cards for transfer rate reasons doesn't make much sense before they actually make use of the UHS-I standard.