Anytime I have ever had a complaint of a slow starting situation, you do a current draw on a fully charged battery, if good, do a current draw while cranking and if in doubt, voltage drop between battery and starter.
In 35 years of being a paid wrench, I have had bad batteries, bad starters, bad cables and a few times, really bad engine bearings. I once had an apprentice go to put his third starter on an engine when I stopped him and asked him if he did those tests. He said he did but didn't know what the data meant. Good batteries, excessive current draw twice, no voltage drop, engine was seized.
It took me longer to use "spell correction" and write this post than it does to do those tests.
Yes, I know, I sound like a smart ass know-it-all again. Sorry.................