I have a 25mbps internet connection. Scenario 1: no wired device connected to the router. In this case I get around 5mbps of internet speed on my phone which is wirelessly connected to the router.
Scenario 2: A laptop is connected to the router using ethernet wire. In this case I get around 20mbps of speed on my phone which is still connected wirelessly to the router.
Wireless spectrum is shared. I posit your laptop was making frequent connection to the Internet - and as it has bigger antennas it, in effect, took priority. When this happened your cellphone connection lost packets, which the TCP protocol uses as a signal to slow down - so it did.
Its also possible something "buggy" was going on, like an error related to handling bandwidth use of wide channels. Its also possible (but, I think quite unlikely) that there is something that amounts to QoS on the router WiFi interface.
