Learn English – What does “tell us know what you think” mean

In this context, heavy means 'using a lot of resources' or 'placing a (heavy) load on the computer running the application'.

A single word replacement might be difficult to find: heavy is quite descriptive.

Google does give results for Thus spake Zarathustra backworld. The first one is http://nathanbauman.com/odysseus/?p=2949

The first paragraph of that page says (my emphases):

Zarathustra’s next two speeches in Nietzsche’s book Thus Spake Zarathustra are related in terms of their assertion that the physical world trumps the Otherworld, or, rather, the “backworld” as Del Caro translates the German term Hinterweltlern. The term is very difficult to pin down in English. The footnote to the word in my edition says that it refers to “those who are of, or believe in, a world beyond, a hidden or a back-world, a secret world”; the note also says, rather curiously, that the word has similar connotations to the English word hinterland. I think what is happening is that Zarathustra is saying that those who believe in an Otherworld are being provincial and silly.

This isn't a German-language site so attempting to translate the German Hinterwelt and Hinterweltlern is not going to be easy. "Back-world" and "back-worlds-men" are the literal translations.

But the expression other-worldly does exist in English: "belonging or relating to, or resembling, a world supposedly inhabited after death" [Chambers], or perhaps "a world outside the physical realm."