Also I don't mean creating actual thumbnails, just putting the doc in an iframe and scaling down. But yeah at least we shouldn't recreate the doc list when you close a document, and preferably we shouldn't recreate all of the iframes when a document is added or something.
Or yeah, we could render the document to a canvas, that's actually possible. Then we could cache all you want
Personally I find these things with loading documents in iframes or canvases or trying to fix night mode quite fun to implement, it's more the surrounding changes to interface and filesystem code that I don't always look forward to