On my (early SSD) laptop I normally choose to put everything in an encrypted LVM because I'm a bit paranoid. :) I assume that all of the great performance features of the file systems such as ext4 assume a lot about the distribution of data that simply isn't true when there's an encryption layer on top. To that end, what is the best file system on top of a layer of encryption? Is it JFS since it "does less" and there for is computationally less expensive?
It supports a bunch of modes of operation, some of which impact performance more than others, and all of which interact differently with the file system. The encryption doesn't necessarily operate using the same block size as the device itself: the XEX-TCB-CTS mode of operation (XTS) deliberately uses blocks that are not evenly divisible by the hardware block sizes, for example.
However, in reality, the overhead from the encryption is so large that it doesn't make any difference at all. I therefore recommend you choose your file system based on reliability considerations.
As maaartinus points out, if your CPU supports the AES-NI instruction set (