If you feel the need to build stuff fast, I suggest just building single scenes and varying all of them quite a bit to show what youre capable of. Building a level usually takes a month or more (at least for me, anyways). The single scene thing really helped me make lots of stuff in under a month, but make sure that you have some real levels made as a lot of places ask for demo levels.
Frankly I think portfolios should be nothing but:
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