Passing along an issue that came up recently on my 05 manual:

Issue: During start either hot/cold immediately or after being off overnight my car would occasionally crank very slowly and almost not start. Other times even right after I would try again and it would crank fine and start.
Checked: Battery held proper voltage, alternator put out proper voltage, cables were tight at battery and starter. I connected a VOM to the battery and did a bunch of start/stops. Noticed the voltages were ever so slightly lower than I would expect.
Resolved: Checking the ground cable from the battery to the car frame it had a small amount of corrosion, so I purchased a new one (like 20 dollars) and removed the old, cleaned the frame where it bolts on and replaced it. Result it the car not cranks faster and the slow crank has not returned.