I have become impressed by the Crusader network throughput and latency tester. It's open source on Github. The Releases page has pre-built GUI and command-line binaries for Windows, macOS, and Linux (which can run on OpenWrt routers).
From the README:
A Crusader test run requires two separate computers, both running Crusader: a server that listens for connections, and a client that initiates the test.
A Crusader test creates three bursts of traffic. By default, it generates ten seconds each of download only, upload only, then bi-directional traffic. Each burst is separated by several seconds of idle time.
The Crusader Result tab displays the results of the test with three plots of throughput, latency, and packet drops.
I especially like Crusader because it makes it straightforward to measure Wi-Fi latency: Run the Crusader server on a computer on Ethernet and run the client on your laptop over Wifi. You'll see the throughput and latency of the wireless connection without interference from your ISP link.
Documentation on the main repo has not caught up to the software; read the latest docs here.