Intermittent slow crank when coil wire is plugged in to the dist

check earth strap from motor to chassis/mount back plate. i.e make sure the chassis end is bolted on on the chassis side of the rubber insulator
check the battery + connector. if its a clamp to wire fitment, rather then a soldered and crimped original check its all good under the clamp
set initial timing to 10 BTDC and see if it starts ok

you have a current flow issue rather than a voltage issue by the sound of things.....

you symptom is identical to what i had when i had failed to connect up the earth strap after an engine out job

voltages all look good
current however isn't because in my case we are accidentally expecting the starter motor to find a route back to the battery through the coolant and radiator and the throttle cable as it passes through the fire wall ferrule.

you have found by taking the ignition load off the system, which will also be earthing via the same restricted route, that it spins a bit faster.... i.e you took an extra 3-7 amps out of the equation and the starter had more to work with.

Its a guess :) not everyone is as clumsy/forgetful as me..... i only noticed because it was dragging on the ground... :) and this is of course the first time i'm admitting this in public

Dave