Have you seen this? I'd recommend to have look:
Maybe a few things can be checked in advance (before tracking things down to disk level):
- double check that no background jobs are running and stealing CPU or IOPS
- with four clients, is the network traffic well balanced across the four Isilon nodes?
- are the actual NFS read/write sizes large enough for 128K? (server and client negotiate a match within their limits.)
- is the random access pattern really in effect?
- for 128K reads, one could also try the concurrency pattern...