6v battery cables

2. Yes all your connections should be "clean, bright and tight".

Still, even with all that, your tractor starter is not going to spin as fast as your Dodge truck. Just the nature of 6 volt systems. The fact that it spins slower than what you're probably used to with your automobiles doesn't necessarily mean there's a problem. If you've taken care of items 1 and 2 above, and it still seems to spin slow, but it still starts then I'd say there's no problem.

An easy check would be to run jump leads to and from the battery and starter circuits and check out if voltage drop is same or less when operating starter. Like, if starter turns quicker, there may be a problem in the connecting wires. If it makes no difference, it may be a lazy starter, assuming battery is up to the job. You can also check this with another 6 volt jump battery and the jump leads. Just be careful with jump lead connections around batteries as any spark can cause a battery to explode!
Be safe and regards, RAB

A 6 volt starter is always gonna sound like it's turning over slow, because you're so used to hearing modern starters turning over so fast. But invest in big cable, new terminals, and shine & tighten all your connections, and you'll be in great shape.