Video and audio in slow motion

You cannot. After effects used to be able to handle that, but no more. It's old and slow. Nuke nc or Davinci Resolve can, and they're free, though you'd need to learn node based effects, which can be clunky IMO. If it seems rude for me to say tbat here, I told the devs this would happen if they didn't start on a replacement in 2018 or something, and here we are, with 3 years of work to add colored keyframes, making renders take several times longer to render on the hardware they advertise working with it, and multithreaded versions of the same effects in other adobe programs, not this one.

You should theoretically be able to load it into RAM the first time watching, but the UI is basically all calculated on a single CPU core and the program doesn't really use GPU, and even the best computers only have enough RAM for like 10 seconds of footage at most, so....

I just use my old CS6 version. It previews faster.

I think you've fallen for a common misunderstanding about using After Effects.

I've made a video explaining about that and the steps you can take to improve it:

https://youtu.be/tdaT81eVbcQ