Got a bit more from http://nfs.sourceforge.net/nfs-howto/performance.html:

5.7. Memory Limits on the Input Queue
On 2.2 and 2.4 kernels, the socket input queue, where requests sit while they are currently being processed, has a small default size limit (rmem_default) of 64k. This queue is important for clients with heavy read loads, and servers with heavy write loads. As an example, if you are running 8 instances of nfsd on the server, each will only have 8k to store write requests while it processes them. In addition, the socket output queue - important for clients with heavy write loads and servers with heavy read loads - also has a small default size (wmem_default).

Several published runs of the NFS benchmark SPECsfs97 specify usage of a much higher value for both the read and write value sets, [rw]mem_default and [rw]mem_max. You might consider increasing these values to at least 256k. The read and write limits are set in the proc file system using (for example) the files /proc/sys/net/core/rmem_default and /proc/sys/net/core/rmem_max. The rmem_default value can be increased in three steps; the following method is a bit of a hack but should work and should not cause any problems: