Macbook Pro suddenly becoming slow to boot up

My specs: Mid-2014 MBP 15" Retina, 16Gb, 2.5 GHz intel core i7, 500Gb SSD, running 10.9.5

Yesterday my Mac booted up to the password screen in about 10 seconds. This morning, having changed nothing whatsoever, I got the spinning gear for well over a minute, maybe 2. I repaired permissions, downloaded the latest OSX security upgrade, and still it's taking far longer to boot than yesterday. I've owned this from new, and it's never shown any sign of this problem before. Any ideas? Thanks