[6cxs2hd6]

I think your point is broader. It applies to spreadsheets, Prolog, SQL. A declarative system needs to be "contaminated" by non-declarative pragmas for real-world use. (I think OP made this point.) To do something faster, or to do it at all.

SQL is a good example. You can get the desired output from a variety of queries... some of which might complete an order of magnitude slower than others. At some point you will need to peel back the "what" abstraction and consider the "how" -- what execution plan will the engine make for a given query.

Regardless, it's still a win. Even if it is no longer purely declarative, you still have leverage. i.e. You're expressing the solution concisely and with satisfactory performance, both.