Tempo is a term used to indicate the advantage gained when a player is able to play more or stronger cards in a shorter period of time due to efficient resource allocation.It is sometimes defined as the means by which a player gains additional options or decreases the options possessed by the opponent by means not directly pertaining to respective numbers of playable cards.
Some forms of tempo are
Mana Acceleration -Players generally develop their mana at a set rate ; accelerants allow players to speed up this process and have quicker access to more mana. This may come in the form of additional channeling cards put into play, and other cards that remain in play and generate mana (ie. Most spawn points and Familiars.) , or cards that generate a one-time boost in mana (Currently none in print).
Mana Drain - Conversely, mana denial cards cause opponents to lose tempo by setting back their mana development and thus limiting their options. Examples of mana denial cards Mordok's Obelisk, and Suppression Orb.
Efficiency - Some cards have stronger effects than other cards costing the same amount of mana. For example, Gray Angel and Guardian Angel despite costing the same amount of mana, the gray angel is overall a less useful creature. Playing efficient cards can generate tempo if they render an opponent's weaker cards irrelevant or substandard, restricting the options available to that player.
Extra Actions - A few cards, such as spawn points, and familiars actually allow a player to take additional actions. This generates tempo in many ways, usually by granting the ability to play extra creatures or cast extras spells, or attack.
Punishment - In some situations, it is possible to counteract tempo generated by the opponent. This generates tempo if it makes the opponent's expenditures useless and therefore inefficient. For example, a player may pay a large amount of mana to attach a powerful enchantment card to a creature or object. If that player's opponent responds by destroying the creature or object with an attack, the player who paid the enchant cost will have wasted the mana and lost tempo.
The temple build gains tempo form the free attack it makes each round and the fact it is a perceived threat. Thus most player will commit manna, creatures and actions to destroying the temples. The main problem is the is the expected response so you have played into the other players plan.