Hi,
Depending on the router and how much free space you have in your router, you should be able to install speedtest-cli.
On the gui plugins look for installing python3 and python3-pip.
Then go on the terminal and do:
ssh root@192.168.8.1 and do the following commands:
pip3 install speedtest-cli
speedtest-cli --no-pre-allocate
It works on GL-AR750S and after installation you should have around 80MB free.
You have also a kind of speed measure on the GUI for the clients but I guess this is what you are looking for.
Regards.
Victor