why is my macbook running slow?

No programs open, macbook plugged into power source - yet when I view videos on youtube and other sites the audio runs fine and the visual is very choppy and delayed. How do I fix this?

Also I feel like my Macbook itself is running poorly. Is there some kind of clean up I should do? I have had it 18 months now. I love it but it certainly used to run a lot faster than it does now.

Thank you for your time,

Elle