Try it.
It varies simply depending on the quality of the connection between the master socket and the one you want to use.
Remove all othe variables, grab your laptop and plug it directly into the modem. Do a speed test 3 times to avoid any random spikes / dips effecting your outcome and then immediately do the same on the second socket.
It will be important to test at the same time of day to avoid traffic shaping or contention on the line effecting the results.
If it's the same, job done. If not, maybe investigate replacing master socket with a built in filter and rerunning the extension with some network cable.