Whip 1 cup heavy whipping cream to stiff peaks. Set a small amount aside in a separate bowl for topping; refrigerate the rest until needed.
Zest 1 lemon to get 1 tablespoon zest, using only the bright yellow skin (avoid the bitter white pith). Juice lemons to get 1/2 cup juice.
In a mixing bowl, combine room-temperature cream cheese, sweetened condensed milk, lemon juice, lemon zest, and lemon extract. Mix until smooth and creamy. (If the cream cheese resists, beat it alone first, then add the rest and keep mixing until smooth.)
Gently fold about 2 cups of the whipped cream into the filling, working under and over until evenly combined and fluffy.
Spread the filling into the graham cracker crust and smooth the top. Garnish with a lemon slice in the center if desired.
Cover and refrigerate at least 3 hours, or overnight, until set.