Yesterday I looked on this excellent forum for any ideas that may explain poor cranking and starter performance that rather suddenly appeared when I tried starting in the morning after a fairly cold night (around 6deg.C 43F) a few weeks ago. I first assumed that, after 5 seasons my LiPO battery was starting to age and with the cold and higher oil viscosity that this was the problem so I bought a new battery. The effect seemed to be temperature related; always being a problem when cold starting. The new battery didn't cure the problem; hence my looking at the forum. There didn't seem to be a dedicated topic focused on exactly this problem, hence this new specific topic, but by various searches i did find a few ideas, even one that reported a similar problem because they had put in 20W-40 oil rather than the recommended 5W-40 oil and this became a problem on a cold day.
In fact I made a list of all the possible options and there are lots, (battery, starter, starter contactor, connections etc. including the possibility of the magnets becoming demagnetised (on a Ferrari forum), but obviously the most likely was a poor connection so I stripped down all the connections between battery and starter, including the heavy duty earth cable that fixes to the gearbox casing, cleaned them, added corrosion protective grease and reattached but really didn't see anything that gave me the slightest concern.
However, on dismantling I did take off the insulated spade terminal that supplies the voltage to the starter contactor and it didn't feel a tight push-on. I used pliers to compress the jaws of the female insulated terminal, cleaned up the spade that emerges from the starter contactor next to the large +ve starter terminal and reattached. Problem solved! Obviously high resistance to the contactor coil can cause the contactor to pull in rather weakly, leading to high resistance in the main starter contacts and a weak start. I am putting this experience out just in case anyone has a similar problem. It might be useful for others to add their own experiences associated with solving the problem of the starter motor working but having difficulty in cranking the engine as quickly as normal.