The popular Network File System (NFS) protocol is 30 years old. The latest version, NFSv4, is more than ten years old but has only recently gained stability and acceptance. NFSv4 is vastly different from its predecessors: it offers a stateful server, strong security, scalability/WAN features, and callbacks, among other things. Yet NFSv4's efficacy and ability to meet its stated design goals had not been thoroughly studied until now. This paper compares NFSv4.1's performance with NFSv3 using a wide range of micro- and macrobenchmarks on a testbed configured to exercise the core protocol features. We (1) tested NFSv4's unique features, such as delegations and statefulness; (2) evaluated performance comprehensively with different numbers of threads and clients, and different network latencies and TCP/IP features; (3) found, fixed, and reported several problems in Linux's NFSv4.1 implementation, which helped improve performance by up to 11×; and (4) discovered, analyzed, and explained several counter-intuitive results. Depending on the workload, NFSv4.1 was up to 67% slower than NFSv3 in a lowlatency network, but exceeded NFSv3's performance by up to 2.9× in a high-latency environment. Moreover, NFSv4.1 outperformed NFSv3 by up to 172× when delegations were used.