engine cranks very slow ?

So what I would do first is try to bar the engine over with the spark plugs out, it should have a fair amount of resistance, if you cannot rotate the engine over there is internal damage to the engine. Also, check the positive battery cable and the ground to the engine, these must be in good shape for the engine to crank over strong enough to start. Please use this video to help.

https://www.youtube.com/watch?v=oN42gZN6gOE

Please upload a short video in your response of the problem, so I can see what's going on. Ken