For a typical site on a server which is not under heavy use the difference should be negligible - certainly nowhere near 500ms. That said, generalization is difficult and very site/application dependant.
Most of the overhead is not the reading/writing to/from the database, but the additional overheads of parsing the code through php - especially when the database contents are manipulated/parsed - eg adding a header and footer, searching the returned page for placeholders (eg shortcodes in WordPress), complex designs which require multiple database queries, access perms etc.
That said, computers are unbelievably fast at these kinds of operations, and a filesystem is just a kind of non-sql database - in fact I'd go for an optimized database based solution on SSD over an optimized static html solution on hdd on a system which is memory constrained.
Its worth noting that - on a lightweight database/static page setup overheads relating to Internet traffic - eg DNS lookup, https negotiation will be a far bigger component of how long it takes to load a page then actually getting the page.
Its also worth commenting that many caching modules/plugins are a hybrid system which renders the static page to disk and whenever its changed in the database recreates a static page which is given to users on request.