Well, give it a try and install the app speedtest.net (Ookla) on your Android Smartphone. It gives you ping latency, upload and download bandwidth to the nearest test server.
Just to be sure to understand whether there’s something which looks unusual.
On the OSMC device you could do
sudo apt-get install speedtest-cli
and simply run
speedtest-cli
and compare the values with the one on your Smartphone. If they are both nearly the same, the issue is behind the Pi. If the Pi is much faster/better than the Smartphone then it’s obviously the Wifi connection which throttles things.