Slow crank

You are working with a lot of unknowns here, so be careful what you assume. I provided you with a common sense approach in post #5, as an example of possible problem.

It is a new engine; might be tight & never spin fast with the thickest of cables. Measuring voltage drops across the cable probably doesn't tell you much unless you have data on the starter. Some starters draw less current than others & so the voltage drop will be less & they may crank ok.

What I would do: to get it started, connect another 12v battery in series so that the starter gets 24v. Be careful to maintain 12v to the rest of the electrical system. Engine should start, loosen up with some run time. Once loosened up, re-evaluate the starting problem, if it still exists.