One of the most important aspects of designing a deck is understanding the deck's "speed". To understand deck speed we must understand some fundamental concepts of Magic.

Fundamental Rule #1: Cards are a limited resource.

Each player starts with seven cards and draws one per turn.

Fundamental Rule #2: As time goes on more powerful cards can be played.

This is true because more powerful cards cost more mana and mana becomes available in higher amounts each turn.

From these two rules opposing strategies form.

A fast deck aims to spend its cards early and often to quickly commit early power to the table. It hopes to overwhelm the opponent before he can use his slower but more powerful cards.

A slow deck aims to negate early attacks and win a longer game through more powerful cards and card advantage.

Generally, the player with the faster deck needs to win the game before his opponent's more expensive and more powerful cards can be played. If the player with the slower deck can survive long enough to cast his high powered spells then he will probably win because both players get the same number of cards and his are stronger or generate card advantage.

It's very important to know your deck's speed. Are you trying to overwhelm the opponent and end the game quickly or are you trying to survive long enough for your big threats and/or card advantage to take over? "Aggro" decks are fast decks. "Control" decks are generally slower. "Midrange" decks have to be able to defend against faster decks but also attack slower decks before the game goes too long.

Know your speed. It's critical not only for deck design, but also for making the right decisions while playing.

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Be sure to check out this classic article on the topic:
Who's The Beatdown? - Mike Flores