Rsync vs Bluetooth
Spoiler : Sometimes the solution is so simple.
I was trying to copy about 20GB worth of data from my wired-connected Pi 2 server to my wirelessly-connected Macbook when I noticed it has been averaging about 100KB/s. There is no way I am going to wait 72 hours to complete.

Like every other geek, I get to work immediately.
I know using Pi as server isn’t the most ideal thing, but it could be one of the most practical solutions if you are well aware of its technical limitations. After all its a RM150 computer taking instructions from RM20 microsd with RM30 power supply. I should be getting about 4MB/s on average or at worst no less than 2MB/s so needless to say, something must be off here.
So I try to initiate the transfer via ssh. It gets even worst at 60KB/s. Then I read about NFS should give better performance on networked *nix pc to pc transfer. That brings me to 120KB/s. So something is right, for now.
I rebooted the Pi, close all the apps on my Macbook with hope it will get any faster but no, it did not went further.
Then I switch off the mouse off because I kept getting low battery popup. And boom - 2MB/s on the spot! So I found the culprit - signal interference.

SO I killed Spotify, disconnected the bluetooth speaker that sit across the room, turned off the bluetooth on my Mac and voila - I am now cruising at 5MB/s again.

I did not notice this kind of behavior before because I was using a wired mouse on my previous Macbook Pro. When I switched to 12inch Macbook, that brought me to the world of wireless and dongles. So things like radio interference virtually did not exist before, and even if it did then it was not so severe that I would more likely to notice it.
Conclusion of the story? World is wireless is still sucks. And I should keep my old Macbook Pro.