Line a 7-inch springform pan with parchment on the bottom. Wrap the outside tightly in foil.
Stir graham crumbs, 2.5 tbsp sugar, and melted butter until evenly moistened.
Press crust into the pan, slightly up the sides. Freeze 10 minutes to set while you make the filling.
Beat cream cheese on medium until smooth and fluffy, about 1 minute. Scrape the bowl.
Add 0.7 cup sugar and cornstarch. Mix on low until combined and silky, then scrape the bowl again.
Blend in sour cream, vanilla, lemon zest, and salt on low just until smooth.
Mix in eggs and yolk one at a time on low, stopping as soon as they disappear. Tap bowl to release bubbles.
Pour filling over the chilled crust. Smooth the top and tap the pan a few times. Cover the pan loosely with foil.
Add 1 cup water to the Instant Pot. Set in the trivet and make a sling with a long strip of foil.
Lower the pan onto the trivet. Seal, cook on High Pressure for 35 minutes. Let pressure release naturally 10 minutes, then quick-release any remaining steam.
Blot the lid if wet, uncover the pan, and cool 1 hour on a rack. Chill at least 4 hours, preferably overnight, before slicing.