What are the best settings to optimize performance and RAM Preview?

Your computer specs are probably going to be the primary factor here, but since you didn't include those, here are some tips within AE itself:

- Understand that After Effects is not a video editor, and preview works differently here. An editing app wants to show you frames as quickly as possible. AE cares about accurately showing you ALL the pixels, and that means more processing time, especially as you stack up additional layers and effects. 

- Make sure imported images and videos are an appropriate size for your composition. If you're working with a 15,000x15,000 px image in a 300x300 comp, you're wasting a lot of processing. Reduce the size in Photoshop, etc. before importing. Do not work with compressed media (like .mp4 video files), as your machine will actually need to de-compress exach frame before displaying it; re-encode any such files into a good intermediate format, like ProRes. 

- Make sure your disk cache (Preferences > Media & Disk Cache) is set to a fast drive (ideally an internal SSD/Nvme drive) with plenty of free space.

- Within the Preview panel, reduce the resolution and/or skip frames. Note that this resolution is actually independent of what your Viewer is currently set to, so you can view/work at Full, but have your preview set much lower if desired. You can also assign different settings to different shortcuts, so Spacebar could be quarter-res and skip to every 5th frame, while NumPad 0 could trigger a full-res preview with every frame displayed. 

- Depending on your hardware, you can try going into File > Project Settings > Video Rendering and Effects and toggle whether you're rendering with GPU or not, to see if that has any effect on your performance. 

Let us know if you're still having trouble!