El segundo. El concepto implícito en la construcción "it is [adjetivo] for [cosa]" es que, normalmente, [las cosas] no son [los adjetivos].
The second. The concept implied by the construction "it is [adjective] for [thing]" is that, normally, [things] are not [adjective].
Also, we would say "something", not "somewhat" here: "somewhat" = "un poco". And we'd use "despite", not "even though": "even though" can only introduce a verb phrase, not a noun.
También, diríamos "something", no "somewhat" aquí: "somewhat" = "un poco". Y usaríamos "despite" en lugar de "even though": este último solo puede introducir una frase verbal, no una nominal.
There is nothing wrong with this sentence. It makes perfect sense. Here is an example context: There are two pieces of scientific equipment. Piece A is lightweight and very efficient. It performs its mission quickly and doesn't use much energy. Piece B looks bulky and is heavier than piece A. So one scientist tells another, "I understand that it looks inefficient, but it actually uses only a little more energy than piece A and performs the mission quickly, but not as quickly as piece A. It is efficient for its weight."
Someone looking at piece B thinks that it would be very inefficient because it is heavy. But if you take a performance measure (speed, energy use, etc.) and divide it by its weight, the result is better than you might expect.