How can I find the bottleneck in my media server setup?
It could be


other wireless devices on your network using wireless bandwidth
a wireless device connecting at a speed that is forcing other connections to a lower-speed protocol
wireless devices not on your network but using an interfering channel
another process on the Mac Mini using network bandwidth


Signal strength and signal quality do not always go hand in hand. Think about somebody shouting in a tunnel—the volume is there, but the clarity will be drowned out by an echo.

One test would be to set up a new SSID and password and connect only the Apple TV and your streaming computer to it (temporarily) and see how that goes.