If you want something small and speedy, Simple Automation Rules (formerly called Simple Lighting) is probably your best best. Motion Lighting is a good second. I'm not sure which is smaller or faster but suspect both are so close the difference is likely to be negligible.
I did wrote a custom app for my lighting, originally to handle my "dim before off" desire and my preference for only using some bulbs in multi-bulb rokms/fixtures. I've just released an expanded version to accommodate many community requests:
I'm not sure if it counts as "lightweight" anymore, but it probably beats a rule, and I prefer "regular" apps whenever possible (one reason I wrote this). I've thought about writing a basic motion light app, open-source and not necessarily offering anything a stock app already doesnt, with the idea that people could take it and customize it however they want...