Is the engine trying to seize? Can you turn the crank by hand. If you remove serpentine belt and spark plugs does the cranking rpms speed up? It should.
Use digital multimeter across battery posts, note reading. Have helper turn key to crank, watch the reading, how far does it drop? If it drops way below 9 volts, it's usually the battery or battery cables. If you think it might be the starter motor, you can remove starter motor, lay it on the ground, hook up jumper cables properly from charged battery to starter motor, If the motor still spins slowly, I suspect the starter motor. Just my opinion. Below is diagram for starter, voltage and grounds are important. I see the relay in diagram, but, I don't think that is the problem, at the moment. I don't have location of relay, sorry.