Bluetooth and WiFi interfering with one another since Yosemite Note: The only thing that changed in my setup is installing Yosemite.

Since installing Yosemite, I'm having a somewhat peculiar problem: WiFi works fine, and BT works fine. But WiFi + BT working together at the same time is a different story. 

I first noticed this yesterday. Two things happen, and the second is more prevalent:


I was using Apple's Magic Mouse + wireless keyboard, and began downloading a large file. Once this happened, mouse movement began being really slow, as if I'm trying to move the mouse on a very uneven surface. Pausing the download would bring the mouse's movement back to normal.
But then something else started happening: Turning BT on would effectively kill my internet connection. From the moment I turn on BT and start using mouse &amp; keyboard, my download speed would go quickly from 1.3MB/s to 500B/s (yes, Bytes), and die slowly from there. When this happens, my mouse movement is just fine. Upon turning BT off, download speed shoots back up to 1.3MB/s.


So to me it seems like BT and WiFi are "competing" with each other, like they're sharing the "same lane" when they shouldn't. Either the normal WiFi speeds will make my mouse move really slowly, or, if my BT is working fine, then my WiFi download speeds will be abysmal. This seems to only happen (or be noticeable) when I'm downloading something like a big file.

I've tried many solutions — PRAM, hardware tests, etc..nothing seems to have helped.

WTH?

Note: This question isn't about the (seemingly more common) WiFi dropping issues, please don't hijack it.

Update 1: There seem to be more people having this issue.

Update 2: Problem persists even after updating to OS X 10.10.1.

Update 3: Over 5 months later, and Apple still haven't fixed this.

Update 4: (29/10/15) One year later (on the latest OS X El Capitan), and my rMBP is still suffering from this issue. Work perfectly on their own, but together they get very challenging. For those who suggested to just get a 5GHz router — I appreciate your responses. However, I don't think I should be obligated to buy a new router just because Apple messed up their software. This is no fix for the underlying issue that is clearly still there for many people. The answer selected as "correct" was auto selected since this was a bounty quesiton.
Bluetooth and IEEE 802.11/b/g/n (WIFI) use almost the same frequency bands:


Bluetooth: 2.402 - 2.480 GHz (79 channels)  
WIFI 2.4 GHz (IEEE 802.11/b/g/n): 2.4 – 2.4835 GHz (11, 13 or 14 channels depending on the country)


Usually Bluetooth uses frequency hopping and changes the possible 79 x 1 MHz bands 1600 times a second to avoid disturbances while WIFI uses fixed channels.

Obviously something went wrong developing the latest Bluetooth- or WIFI-drivers.

The only workaround until Apple gets their drivers fixed is using dual band wireless access points. Those devices use two different frequency bands:
2.4 – 2.4835 GHz and 5.18 - 5.825 GHz (with some gaps).
Connecting to WIFI 5 GHz (IEEE 802.11a/h/n) will not interfere with Bluetooth.

Almost all Macs sold since 2006 support 802.11 a/b/g/n.

Apple's answer: Wi-Fi and Bluetooth: Potential sources of wireless interference

BTW: Not only typical RF-based devices (like wireless phones, etc) but also bad-shielded USB3-devices and cables interfere with WIFI (2.4 GHz) and Bluetooth.