Very huge fall of FPS near blinking light with smooth lighting (no matter "minimum or maximum").

Performance near blinking light with smooth light on becomes very bad.
For expample on screenshots, when redstone clock off with smooth lighting (40-80 FPS), when it is on with smooth lighting (5-10 FPS), and on without smooth lighting (70-100 FPS)

My Computer:

Processor: AMD Athlon(tm) II X4 635 Processor (4 CPUs), ~2.9GHz

RAM: 4GB

Video: Nvidia GeForce GTX 560