Keep in mind there was no such thing as standard written English well understood across the whole country until relatively recently; spoken English even less so.
Even today, someone in rural Northumberland would not necessarily clearly understand every word spoken by somebody from rural Devon.
Personally, as a reasonably well educated native speaker of English, most (>95%) of Shakespeare is clear to me.
Chaucer (late 14th/early 15th C) starts to be a struggle.