[skidlids]

I'd try shorting out/bypassing the starter solinoid and see how quickly it turns over then,

As Martini says it could be high resistance which could be down to a well worn solinoid, which in effect is a metal busbar that shorts the two studs together that the battery cable and starter motor cables are connected to
and with the current drawn involved it causes an arc which over time causes the surface to become pitted and carbonised, hence becoming a poor (high resistance) connection

Also to be considered are the stte of the Starter motor brushes and the condition of the Starter motor idler gear pinion which as Bradders found out can cause big problemswhen starting