Add softened cream cheese, sweetened condensed milk, lime juice and 2/3 of the lime zest to a large mixing bowl, and beat on high until smooth and creamy (about 4 minutes).
Transfer to a graham cracker crust.
Sprinkle reserved lime zest in top, and refrigerate until filling is set, about 3 to 4 hours.
Serve chilled, and garnish with whipped cream extra lime slices, if desired.