Common causes of a slow engine cranking are degraded or defective battery, faulty electrical connection, a charging system problem, a bad starter or battery rundown when parked.
The cause of slow engine cranking is usually a bad battery that is not holding it's charge or not taking a charge.
When you turn the key to start the engine the engines starter motor needs a lot of amperage to operate and turn the engine over to start.
So you need a good battery for that to happen.
If the battery is bad or older than 5 years then it could be the cause.
If that's not the issue and you have a fully charged battery or you have tried Jump Starting the vehicle and the engine still cranks slow then it's likely that your starter is going out.
Starters can eventually go out and start dragging when you try to start the vehicle.
When the vehicles engine starter motor starts dragging it draws more amps when trying to turn and start the motor.
I would first make sure the battery is fully charged and if it's not fully charged charge it up and then try starting the vehicle again.
If the battery keeps going low then you need a new battery.
If the battery is fully charged up and still the engine is slow to crank over then your starter motor is likely the culprit.