Q:

Does WiFi on the Pi 3 reduce the Ethernet bottleneck when using USB device?

We all know that on Raspberry Pi 2 and previous models, the USB and Ethernet bandwidth is shared, so when we transfer data over Ethernet to a USB device and vice-versa we are limited by this fact.
Since on the Raspberry pi 3 we have WiFi capabilities, can we use wlan0 to reserve all the bandwidth to USB ports?

A:

From the looks of it, yes. From the info I've read, the BCM43438 is connected to the SoC in two ways: 

The WiFi functionality uses the SDIO interface
The Bluetooth functionality is connected via UART

Based on this, we can safely say that it completely bypasses the USB interface so you can go ahead and use the USB ports to the fullest without affecting networking speed and vice versa.
tl;dr: The WiFi/Bluetooth chip doesn't use USB.
Source: Post by a Raspberry Pi engineer: (Link)

