Short description of error
Take any prodution render with CPU, just activate osl in the render tab, it will render " to 4 time slower, without using it in the scene. Compared to GPU render wich is already 5 times faster than a cpu render, it is = 15 to 20 times slower. OSL is unusable with this bug. Note that other renderer with OSL don't have this speed hit.
Exact steps for others to reproduce the error
With CPU and 16x16 tiles, render this scene: http://download.blender.org/demo/test/pabellon_barcelona_v1.scene_.zip
then just activate osl, render again and compare times. although there isn't any use of OSL in the scene, the render times crawls down.
(you can also lower the samples to 10 to make it faster for both render)