I've seen a few starters that had a failed oil seal, which resulted in oil getting inside and gumming up carbon particles in the slots between poles. My 07 with over 100,000 miles was starting slow and sometimes wouldn't start when hot. A simple test is to use a clamp on DC ammeter on the big wire that goes from batter to starter relay to starter. If you see 100 amps or more, you're starter is toast.
I pulled my starter, which is a big job because the throttle bodies have to come out to get access to it. Sure enough, the oil seal had failed. I found oil seals online (Yamaha doesn't sell them, although they do sell other starter parts) and rebuilt the starter. Works fine now. Going from memory here, but on the bench the failed starter pulled over 80 amps and the rebuild resulted in less than 20 amps. I have a post somewhere on this forum about my rebuild.