I have frontend in js where you can type in iperf command and then POST it to backend in java. Then use Runtime.getRuntime().exec() to check users bandwidth, is it even possible or it's just not how iperf works. My guessing is it is only possible to check server's bandwidth.
TL;DR
I want to to make online speed test using iperf
You can try the experimental Network API https://developer.mozilla.org/en-US/docs/Web/API/NetworkInformation/downlink