If you have a USB 3 peripheral and are experiencing Bluetooth or WiFi troubles, you’re not alone. According to Intel (via Rosyna Keller), this is a known issue:
[The] noise from USB 3.0 data spectrum can be high (in the 2.4–2.5 GHz range). This noise can radiate from the USB 3.0 connector on a PC platform, the USB 3.0 connector on the peripheral device or the USB 3.0 cable. If the antenna of a wireless device operating in this band is placed close to any of the above USB 3.0 radiation channels, it can pick up the broadband noise. The broadband noise emitted from a USB 3.0 device can affect the SNR and limit the sensitivity of any wireless receiver whose antenna is physically located close to the USB 3.0 device. This may result in a drop in throughput on the wireless link.
Intel’s white paper on this issue is nearly four years old now, so this may be old news to some of you; I, however, would never have thought to try unplugging USB devices as a troubleshooting step for poor WiFi performance.