nfswatch is an indispensable diagnostic tool for heavily used NFS servers. Unfortunately, it only supports NFSv3 and is blind to all NFSv4 traffic. For several years we've been looking for a NFSv4-capable replacement in order to be able to diagnose IO issues on our file servers. After several failed attempts we finally discovered the nfs tapset for systemtap that readily provides the data needed for an in-depth inspection of an NFS server. nfstop consists of a python script that launches a systemtap probe which outputs a JSON that python then parses and displays.
On Debian, you'll need systemtap linux-headers-generic linux-image-amd64-dbg python3-rich. On Ubuntu, it's linux-image-$(uname -r)-dbgsym, see here.
Just run ./nfstop.py. It'll take some seconds to compile and insmod the systemtap module. Currently there are two command line parameters: --seconds to set the update interval and --lines to determine how many hosts/users are shown in each category.