I received an e-mail asking why Ubuntu was mounting NFS file systems with version 4.0 instead of 4.1, I responded I did not know and would need to research it.
What I found was that actually version 4.2 is supported by most modern systems, and according to the manual the NFS client is supposed to negotiate the highest version mutually supported by the client and server but for some reason it will not do this.
I found that if I added vers=4.2 to the mount options on machines which support it (the old centos6 machine does not with the stock kernel and is using 4.1 instead), then when the machine boots it will use that version. This should not be necessary if things worked as documented but that is not the case. Performance of version 4.2 is significantly better so changing this substantially improved the web server performance because it accesses it’s content via NFS.