I'd suggest to look into why it is so slow to load the preview. Increasing the preview timeout is not a silver bullet. Let's say you have queries to an external data source to display something in preview, and that data source is slow. If the query is killed after 20s, then that data source is "freed" somewhat and can serve other queries. But if it continues to run, it can even slow down more and can't complete after 40s. That is why you might see even more timeout with increased timeout threshold
I experienced "Preview Unavailable" issue for some of the pages and found the fix in setting/increasing PreviewTimeout. However I set it to 40000 instead of 20000, and noticed, that it's now even worse than was. Almost every page has no preview now, and those page previews which eventually loaded took way too much time to load preview than before. Is it because I set PreviewTimeout to 40000 instead of recommended 20000?
Also we haven't got this "key" in our Web.config before at all.