Fast Rewards are game mechanics in AFK Arena. The purpose is to grant players the same items that could be obtained from the AFK rewards chest.
Each use of Fast Rewards gives a player 120 minutes worth of AFK rewards. That includes the possibility of getting better rewards such as Mythic Gears and Buffs. The first daily use of Fast Rewards is always free and after that it uses the cost of an increasing price in Diamonds.
While using Daily Rewards many times can be tempting, it can quickly build-up to some bigger numbers. Therefore they should be used sparingly unless a player can obtain Diamonds easily. If you can afford it, Daily Rewards are a good source of Gold, Hero Experience and Hero's Essence.