I just started using After Effects. I have some footage edited together with an mp3 music file. When I click spacebar to play the preview, the video and audio is okay for half a second and then both start going extremely slow. I have 16GB of RAM and an NVidia GPU on a Windows 10 Surface Book. Please help!
please specify the exact version number of your After Effects Software
After Effects is not video editing software. It will not behave like video editing software.
If you want to do video editing, you should use Premiere Pro.
If you want to do motion graphics, compositing, or visual effects to an already edited sequence, feel free to use After Effects. Just know that After Effects is not like Premiere. In After Effects, you have to cache a preview before it will play back at full speed.
Since you're new to AE, YOU HAVE TO get a good foundation first. Just poking at it will make you very frustrated. Go through the Getting Started tutorials here on Adobe's website. Or go through some of the handy resources linked here. (I particularly like Andrew Devis' tutorial series.)