If you lift up the back of the bike, spin the wheel, and detect no unusual drag, it's not the rear wheel or the rear brakes.

If you lift up the front of the bike, spin the wheel, and detect no unusual drag, it's not the front wheel or the front brakes.

If you remove the chain from the chainring, spin the crank, and detect no unusual drag, it's not the BB.

If you take the chain off the derailleur pulleys, spin the pulleys, and detect no unusual drag, it's not the derailleur.

If you do all of those things without detecting unusual drag, but the bike still feels slow, you have mono.