That's the reason for bypassing all the intermediate connections with a jumper cable. If the starter spins normally, or at least better, then disconnect every connection until you find the bad one. The hot side can be checked the same way. Be absolutely sure both the battery posts and inside the cable ends are shiny clean. Scrub them thoroughly with a wire brush.
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!