The page in general is okay. IMO they should actually lead with the second paragraph, which does mention performance, and some things you might want to do with it. They should combine that with the call out that's strangely floating off to the side that states "Mercury is a general purpose language intended to support the creation of large, reliable and efficient applications."

Here, I'll give it a shot:

"Mercury is a general purpose logic programming language that supports the development of large, reliable, efficient applications through static program analysis and a tunable execution algorithm"

From this one sentence I get the what (logic programming) why (for large reliable apps) and how (static analysis, customizable execution). Now I'm interested.