The reason why nfsd consumes 850% CPU is that measurement is per CPU core, and it currently uses more than 8 CPU cores. It does that by running more than 8 threads (LWPs).
Unfortunately the HP-UX ps command does not show the process threads (where Linux and Solaris have the -L option for ps).
You can see the NFS statistics:
Further, if you have tpcdump installed, you can see which NFS client is currently accessing: