Slow video

[Patrick_Tan]

Hi, im new to after effects and just downloaded a preset just today but every time i use a preset on a video like a colour corrector it just makes the video slow down. Can anyone help me thanks!

[Peru Bob]

What are your system specs?  How much RAM?

[Patrick_Tan]

8 GB 1600 MHz DDR3

I'm using a macbook air

[imeilfx]

After Effects is not a realtime payback app and it can slow down (dependant on your workstation specs, footage and/or effects that you use) so you have ram preview. About that you should star here: Preview without rendering in After Effects 

And as a beginner you should first try to learn some basics. AE is not the easiest piece of software and without basics can be hard to do anything.
You will find some good basic tuts here: After Effects tutorials | Learn how to use After Effects CC 

[OussK]

As imeilfx say's you need to have a ram preview to see in real time so you must to cache before playback,

Anyway your RAM is not enough to run AE smoothly