Iperf 2
A means to measure network responsiveness and throughput
... and iperf3: Read about the differences between these programs at https://iperf2.sourceforge.io/IperfCompare.html
Man page: https://iperf2.sourceforge.io/iperf-manpage.html
GPS atomic clock options for RPi:
https://store.uputronics.com/index.php?route=product/product&product_id=81
https://store.timebeat.app/products/gnss-raspberry-pi-cm4-module?variant=41934772764843
Open source time server:
https://github.com/opencomputeproject/Time-Appliance-Project/tree/master/Open-Time-Server