If your vehicle is showing engine cranks very slowly, here are the most likely causes and the parts you'll need to fix each one.
The battery lacks the necessary cold cranking amps (CCA) to spin the starter motor at normal speed. This is the most frequent cause of a slow crank, especially in cold weather or if the battery is older than 3-5 years.
Corrosion or loose clamps on the battery posts create high electrical resistance, preventing full power from reaching the starter. This is highly common if terminals aren't cleaned or tightened properly after recent battery work.
Internal wear in the starter motor (such as worn brushes or a failing armature) causes it to draw excessive electrical current and spin sluggishly, even when connected to a fully charged battery.
If the alternator is failing or its drive belt is slipping, it will not fully recharge the battery while driving. This leads to a progressively weaker battery that struggles to crank the engine.
Using engine oil that is too thick for the current climate (especially in freezing temperatures) creates excessive internal drag. This makes it very difficult for the starter to turn the engine over.
Severe internal engine damage, such as spun bearings from oil starvation or hydrostatic lock from fluid in the cylinders, creates massive mechanical resistance against the starter motor.