I have a Asus N56v Laptop, recently upgraded to Vivid and now my Brightness Controls accessed via (FN+F5, FN+F6, FN+F5) have a dramatic lag. I can wait 30 up to 120 seconds before the keystroke is registered and the system adjusts.
I initially didn't think they were working at all, after spamming the keys it took 10 minutes to finish adjusting. The Volume keys configured the same way with fn+f10 work just fine and quickly.
This isn't the first time I have run into issues with the brightness controls either. They worked perfectly on Ubuntu 14.04 but stopped working all together in 14.10 and now they work but with extreme latency.