When you say you tested at the starter terminal, do you mean at the starter relay, the starter motor, or at the switch on the bars?
A slow turning starter could be a few things...here's what I'd look at:
1) weak power-- check you battery, if fully powered, then test it at the relay, there are two main wires (large black ones with boots)...one comes from your battery (the one on the left, should be able to follow it directly to your positive side on the battery), when you press the button, the relay sends power to the other one (on the right) and down to the starter motor. when you press the button, you should have 12+ volts going to the starter motor. If not, then you will need to see why you're dropping power between the switch/relay, etc.
2) bad grounds... check all the grounds/connections (the starter motor mounts are it's ground).
3) Bad starter motor...they do need to be cleaned from time to time. polished/new brushes, etc. Also, the oil seal can leak and oil gets into the starter motor...over time, this will make the motor spin slower and slower and ultimately stop working (I just literally had this happen to mine. I pulled it out, swapped for another from a member on this board and it works great now. The old one I took apart and poured all the oil out of it...cleaned it up, and works great!
Test the starter motor...Make sure the bike is in nuetral and then simply get a jumper wire from the positive terminal (on a fully charged battery) and touch it to the starter motor terminal (Under the bike..it has a boot on it, and is the only cable going to the starter motor). If the motor is good, it will spin like crazy and then you'll know it isn't the issue. If it spins slowly, it's gonna need to be cleaned.
Hope one of these helps.