Pete52 has it right:

The place to begin when experiencing slow cranking or no cranking is the positive battery cable and the negative battery cables and grounding straps/cables. They should be at least (most?) 2 gauge, but 1 gauge or 0 gauge or 00 gauge are preferred. Most replacement battery cables were/are 4 gauge (appropriate for a 12v system).

Second: weak battery

Third: weak starter