I have this issue too, so frustrating. Before Yosemite my mac would boot perfectly within a couple seconds, after updating to Yosemite the boot time got extended but i didn't complain too much. But the recent update to 10.10.3 is really disappointing, pressing the power on button and the first thing i hear is the chime within the first few seconds but my mac screen will be black with nothing for a good 10-15 seconds before the password screen shows. At first when i got that i actually thought my mac broke and freaked out but i decided to just let it run the black screen and was so relieved it actually works but there is just a delay of the black screen. Seriously apple's softwares getting on my nerves recently, they churn out devices yearly but seems like they don't care about software like they used to, really disappointing.