How can I efficiently build a deck from a box of random cards?
If you're willing to spend a bit of time up front, you can probably "triage" the cards in a way that might make deck building easier.


Grab a large section of cards and sort out the colors you are interested in.
Sort the cards within the colors by the set symbol.  The idea here is that you're more likely to get interesting synergies from cards that are from the same set.
Choose the sets that have the most cards in them, and sort those into lands, creatures, and non-creatures.
If there are still too many cards, you might sort the creatures into ones that share a keyword or troop type.


Now you can start your deckbuilding.  Start with creatures and add ones that look interesting until you have about 25 creatures.  Then go to the non-creatures piles for the sets you used, and find 10-12 that go well with the set of creatures you chose, swapping as necessary.  

This way you aren't having to read every card, and you're more likely to end up with a deck with at least some interesting interactions.  It isn't going to win a tournament, but it will let you get started, and then you can read more cards as you have time and continue tweaking your deck.