I installed OpenCV libraries in raspberry pi for my project "Air Hockey Robot" and I'm using python for video processing. I am using PS3 eye camera for video capture at 60 fps. But it seems that video processing is too much slow. Should I use multi-threading or that's the general problem with Raspberry Pi 3?
Honestly, I don't think 60 fps is achievable, even on a Pi 3. While the Pi 3 is fast for a Raspberry Pi, the processor is still only 1.2 GHz. That said, you could do a few things to help and see if it is achievable with some optimisation; OpenCV itself is in C++ which won't experience such large performance problems as Python does.
You can check to see what the cause of your largest delays are using a profiler like
cProfile for Python; this would help you to determine which of your functions are taking the longest, and which you should focus on optimising.
You could consider using a lower frame rate instead which could increase your processing time per frame, or experiment to see if multithreading helps. Python is often a little bit problematic to thread due to the general interpreter lock which prevents two threads of the interpreter from running simultaneously.