The typical setup is like this.
Modem---(wan)Router~~~~~~Bridge(lan)------PC
or
Modem-----(wan)Router(lan)----------(lan)AP~~~~~~device
---- being ethernet
~~ being wifi.
Depends on what you mean by fastest. That is large files or video streaming need high speed while online gaming requires low latency. Either could be considered for determining the "fastest" connection. For low latency, removing the wifi altogether is the best approach. Ethernet is the guarantee of highest network throughput, but will not necessarily improve internet download speeds.