In a small saucepan over medium-low heat, combine 1 cup water and 1 cup sugar. Stir gently until the sugar completely dissolves, about 2-3 minutes. Remove from heat and let the simple syrup cool to room temperature, then pour it into a large pitcher.

While the syrup cools, juice the lemons. Roll each lemon firmly on the counter with your palm to soften them. Cut in half and squeeze using a juicer or your hands until you have 1 ½ cups of fresh lemon juice. Add the juice to the pitcher with the cooled syrup and stir well.

Add the remaining 7 cups of cold water to the pitcher and stir to combine. Refrigerate until thoroughly chilled, at least 1 hour. To serve, pour over ice cubes in glasses. Add lemon slices if desired.