Real-time Raspbian for Raspberry Pi & Pi2
Untested by me. Claims to respond on the order of tens of milliseconds instead of hundreds.
This is a release of Raspbian with real-time kernel. It is compatible with Raspberry Pi models: 2B, B+, A+, B, A.
It is based on 2015-02-16-raspbian-wheezy with default kernel replaced to 3.18.9-rt5-v7+ kernel and a few additional tunings.
Real-time Linux for RPi2 - Emlid
Github repo here with instructions on how to build: