[MN_Mime]

99 isn't speculative at all. It's best hand.

You've got ~72% equity against a random hand. You're ~50% equity against any broadway (includes bigger pocket pairs). Against 2 any broadway hands, you've still got your 33% equity share. Even adding 2 random hands (5 hands, including blinds), you've got ~25% equity PF.

99, however, often ends up in the grey zone when the flop comes becuase you're often in the unpleasant situation of two overs. You want to limit the field to reduce the chance of a randomish hand catching an overcard; the combinations just kill you. You don't want to give the blinds a free ride.

When you fail to do so and you end up with an overpair, you need to recognize that you're now playing for set value instead of high-card strength. Building the pot early gives you better odds for hitting your set later if the flop is not favorable.

As OP suggested, OOP presents problems and your read will help you decide if you can gain fold equity by representing a bigger hand or releasing.

Headsup against a passive, I prefer pressuring him but his call speaks volumes. He's made a hand (or started with one), but the question is how fragile? I initially missed the passive read. If the read is solid, I might check the turn and check/call the river. If he's just semi-passive, I like OP's line.