That's why I thought of viscosity first
BUT if you're not using the wrong oil then you're thinking that there is some, internal, "pressure load" of some sort that lessens as you crank for awhile? Not sure that can happen......
More likely on the oil side would be the "thickness load" lessens as things warm-up from the friction of spinning. Just for the heck of it, I would at least try
plugging it in once
I'm also not sure you can expect a tired starter to loose it's ability to crank with time and just get slower & slower. I don't think it has to work that way.
My thinking was, (and without being there to hear what it sounds like I'm just spit-ballin here) that if the starter were marginal the initial cranking might not be building enough pressure for the injectors but with some time it could cross that threshold and the engine itself could add to the cranking speed as an injector or two fired a cylinder.
I don't know how "slow" your "slow" cranking is,,,, but, as I said, if it's too slow you're just wasting your time and batteries because it won't even attempt
to start without being over 100rpm.