If it's video only, you can also add a velocity envelope and slow it down near infinitely, but this won't affect the audio at all.
If you need to slow both down past 4x, you can go to 4x, then render that result to a new intermediate file, then bring that 4x file back into your project and slow that down up to another 4x.
Nesting veg's is a lot slower then using a media file. If you have the space, render to a lossless format (I like quicktime png or a png sequence). It will take up a lot more space but work a lot faster when editing and rendering.