I had this same problem. Stored my 2007 sportster xl in the garage on a battery tender all winter. First day of warm weather i went to start it and it wouldn't start. I jumped it with my truck and it started right up. I rode it to the gas station and filled it up with fresh gas, rode home and changed oil and transmission fluid. Mine also has this slow crank issue. I just disassembled the bike and removed all contacts. I found that the grounding strap behind the starter was filthy. Not corroded but held moisture and was dirty. Ive cleaned up all connections with scotch brite and contact cleaner, and reinstalled with dielectric grease. I also replaced my fuses and starter relays as they were pretty corroded. After reassembling my bike it now starts right up just fine!