Try WiFi Analyzer, free in the Play Store.
You could be sharing a channel with a neighbor and not knowing it.
When that happens, arbitration takes over to deal with traffic collisions. Some devices can do ok with lots of collisions, some can not.
If you are overlapping channels with someone here, not only will getting your router on a free channel fix this, it will improve your throughput for you other networked machines as well.
Originally Posted by Digital Controller
Apps do not use specific ports...
Yeah - it's not the problem here, but some apps do use specific ports.