Very Slow Sequential Reads over NFS from an XFS disk in Amazon EC2

I am using tiobench to test performance of an NFS mounted volume, and notice that Sequential Reads are much slower than Random Reads. This isn't the behavior when I run the same test on the disk mounted locally.

Here is a little graph, with NFS in red and sequential reads on the left:

I am exporting the volume on the server like this

/export *.internal(no_subtree_check,rw,no_root_squash)

and mounting with this:

mount -o hard,intr,async,noatime,nodiratime,noacl $NFS_SERVER:/export /nfs 

Additionally I am doing all this in amazon EC2, exporting an EBS volume with the XFS file system (redundant, I know).

I have tried using jumbo frames and various other mount options, but none seem to have much effect.

Thanks for any clues.