I had this problem using a targus Bluetooth keyboard with touch pad, and a dell Bluetooth mouse. Enabling Bluetooth Collaboration was the resolution for me, but only after first disabling the touch pad on my keyboard.
I found the setting for Bluetooth Collaboration in the properties for the network adapter. Commonly, Bluetooth is included with the wireless chips. So to resolve:
Open Network Connections (In windows 8, select the windows key and X , then select network connnections
Select your WiFi adapter (not your Bluetooth adapter), right click and select properties.
In the WiFi properties window, select Configure button.
In the <your network adapter name> Properties window, click on Advanced property sheet(tab).
In the Property Option list, scroll until you find Bluetooth Collaboration.
In the Value option list, select On (instead of auto or Off).
click OK, and close all open windows.
Test.
it looks like this solution doesn't apply to the Surface's wifi-bluetooth chip / driver combination.
This thread doesn't discuss general wifi-bluetooth issues, but only Surface-related.
Changing the state of Packet Coalescing seems to solve the problem temporarily (turn it on if it's off, or turn it off if it's on). I have 3 BT devices connected to my Surface Pro and the mouse would lag horribly whenever anything was playing through the earphones. It stopped lagging once I changed the Packet Coalescing option's state. However, it came back after I did something to trigger it, which I have yet to determine. Ugh.