Theyre both different

_document is to update the html or body tags globally

This will affect all pages

You can add meta data there but its better to add them to your server side page or in layout

Where they can be dynamic or each have their own meta

If I keep some static meta tags in _document file, does it cache those meta tags across all pages.
To improve performance/speed.

It all happens in the server so there is no improvement

At the end of the day, both meta data get crawled by bots and read