Let the frozen peach slices sit at room temperature for 3 to 5 minutes to soften slightly, which helps them blend more smoothly. Break the frozen banana into 4 to 5 chunks before adding to the blender.
Pour the milk into the blender, followed by the Greek yogurt, vanilla extract, and honey or maple syrup (if using). Adding liquid first creates a vortex that pulls the frozen fruit toward the blade for more efficient blending.
Add the frozen banana chunks, then the frozen peach slices on top. Sprinkle the ground cinnamon over the fruit. Placing frozen fruit furthest from the blade allows the blender to build speed before hitting the hardest ingredients.
Secure the blender lid and start on low speed, gradually increasing to high. Blend for 30 to 60 seconds in a high-powered blender, or up to 2 minutes in a standard blender, until completely smooth with no chunks remaining.
Taste the smoothie and add extra honey if more sweetness is needed, a splash of milk if it is too thick, or a few more frozen peach pieces if it is too thin. Blend briefly again to incorporate any additions.
Divide the smoothie evenly between two tall glasses. Garnish with a fresh peach slice on the rim, a pinch of cinnamon, or a handful of granola if desired. Serve immediately for the best texture and temperature.