The simplest way is to run tshark on the client with options to print rpc response times:
$ tshark -i any -f "port 2049" -Y rpc -Tfields \ -E separator=/t -E header=y \ -e frame.number \ -e rpc.repframe \ -e rpc.time -e ip.src -e ip.dst \ -e nfs
The output will be something like:
frame.number rpc.repframe rpc.time ip.src ip.dst nfs Capturing on 'any' 3 x.x.x.x y.y.y.y Network File System, Ops(1): SEQUENCE 4 3 0.000593460 y.y.y.y x.x.x.x Network File System, Ops(1): SEQUENCE 8 x.x.x.x y.y.y.y Network File System, Ops(1): SEQUENCE 9 8 0.000797399 y.y.y.y x.x.x.x Network File System, Ops(1): SEQUENCE
The rpc.time in the reply frame is the difference between sending a request and receiving the reply.
NOTE: the options and output may be different depending tshark version.
Alternatively, you can just collect network traffic with dumpcap or tcpdump and analyze it with the wireshark.