Warmer temperatures may make many crops grow more quickly, but warmer temperatures could also reduce yields. Crops tend to grow faster in warmer conditions. However, for some crops (such as grains), faster growth reduces the amount of time that seeds have to grow and mature.  This can reduce yields (i.e., the amount of crop produced from a given amount of land).
For any particular crop, the effect of increased temperature will depend on the crop's optimal temperature for growth and reproduction.  In some areas, warming may benefit the types of crops that are typically planted there. However, if warming exceeds a crop's optimum temperature, yields can decline.