Slow demos (movies)

Hi, I am having a problem with demos, they are slowed after render (making a movie out of it).

I followed all the instructions on valve developer wiki to make a demo and to render it, I even used the recommended tools (VirtualDubMod), but they are still slow

here is an example: https://www.youtube.com/watch?v=uJAR5EKOxi8&list=UUVf-mU5715manouruDTp-_A

This video got audio lag, but the one I render don’t. Is there any way to fix that too?