I'm curious about how one is supposed to reason about the time and space complexity of declarative programs. I don't work in a realm where CPU and memory costs

I'm curious about how one is supposed to reason about the time and space complexity of declarative programs. I don't work in a realm where CPU and memory costs can be assumed to be infinite (or even cheap). How do declarative programming paradigms typically offer guarantees or bounds on computation cost? I'm assuming that any declarative language powerful enough for general use is expressive enough to represent an NP-complete problem; i.e. : Find a set of booleans X1..XN that satisfy Y logical statements (or prove that there is none possible). Therefore absent opening up the "black-box" of the language, the time and space complexity could be unbounded. With an imperative language, of course, since the programmer is probably specifying the exact order of operations and data structures being used, reasoning about and bounding the cost is usually straightforward.