[franken]

The ignition switch and solenoid have already done their duty once the starter turns.

If the starter cranks slow, it's the big cables that aren't passing the current, poor connections on the big cables, the solenoid contacts are burnt, the battery is dead/discharged, or the starter is the problem. Nothing else is in the circuit driving the motor other than the extra solenoid.

Clean all cable ends of the big cables. Try using one lead of jumper cables from the battery negative and connect the other to the starter body to somewhat bypass the big cable. Try the same from the pos terminal to the big connection on the solenoid. Lots of current capability so be careful not to do any welding...

If the distributor has too much initial advance it can cause a slow crank issue.