Default Advice

Hey there,
I really have a question for you guys. To be honest, I am sick and tired of my galaxy s3 running slow and blocking all the time with the official android release. I want to install something like the pure android experieence (kitkat), like on the nexus 5.However, I have no idea what to do. Can anyone help, or will installing a simpler android (like a stock version) not help with "fluency" of the phone?

Thx in advance