Really?: "Pročs to dělal tak pomalu?"
Yeah, this one is more than a little over the top!
Is it correct?
Or rather ... somehow acceptable, but I do not recommend it.
What’s the normal position for to? Why does it go between jste and delal?
To is an (inconstant) clitic. In this sentence, it follows the second position rule for clitics. Read this: Ordering the Czech Clitics: Introduction