1999 Pontiac Grand Am car gets hot very slowly

Try replacing the thermostat and see what happens

Disconnect the negative battery cable.
Drain and recover the coolant. Refer to Service and Repair.
Remove the exhaust manifold heat shield.

Remove the coolant inlet housing bolts through the exhaust manifold.
Raise the vehicle. Refer to Vehicle Lifting.
Remove the coolant inlet housing stud from the oil pan.
Remove the tire and wheel assembly.
Remove the splash shield.
Remove the bolts from the transaxle-to-engine block brace.
Remove the coolant housing pipe.
Remove the thermostat.
Clean the mating surfaces.