Slow cranking is a possible sign that the engine is tight. Can you turn the crankshaft by pulling on the fan blades with the spark plugs removed? If not, find out why. Take the plugs out, lube the cylinders well with penetrating oil like Marvel Mystery Oil or KROIL, wait a couple of days, and then spin the engine with the plugs removed. Repeat as necessary until the engine spins freely. Extreme load on the starter can load even a new battery to the point that there's no more current left over after running the starter to power the ignition system. Try a quick boost with 12 volts to see if you can increase the cranking speed but don't burn out the starter with extended cranking time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!