Gearing, tires, and geometry make all the difference in the world.

My Transition Sentinel is only geared for mountain biking. It's a terrible city bike. Tons of shock, high torque gears for steep hills, cannot go very fast. But it's insane when you need to climb or descend mountains. It has knobby, 2.4in tires.

My city bike is an ebike, and even though it's a single speed, it's pretty comfortable going between 10-30mph on that gear alone. The battery allows me to haul lots of groceries or baggage (and climb steep hills), and it's tires are wide enough to not get stuck in tram rails or gaps in the concrete road. I have knobby tires to avoid popping tires, but smoother, thinner tires will be more efficient.

Edit: if you have a shock, try locking it out if it has lockout.

I'd also recommend checking out city bikes, such as road, gravel, and upright bikes. There's an incredible amount of diversity, and a downhill mountain bike is about as far from a road bike as one can get. One can roll over a rock the size of a watermelon, the other can coast for meters off of a pedal stroke. Ebikes also are phenomenal as car replacements (or even just as car offsets), but generally cost $1,500+ with tariffs.