Add 2 cups of cold water to the insert of your instant Pot.
Add in the sugar, and whisk to dissolve.
Place the lid on the Instant Pot and set the valve to "sealing."
Cook on high pressure for 4 minutes.
When the cook time is through, switch the valve to "venting" and release all of the pressure inside the pot.
When the pin drops, indicating the pressure has been fully released, carefully remove the lid from the Instant Pot.
Pour in the remaining 6 cups of cold water and whisk.
Add the freshly squeezed lemon juice to the Instant Pot and whisk again.
Transfer the lemonade to a half gallon size jar or a glass pitcher. Cover and place in the fridge.
Chill completely before serving over ice, garnished with fresh lemon slices (if desired).