Since my last update on Yosemite 10.10.3, start up is very slow, does anyone have any help to restore start up speed please?

The bit that is very slow is the time between the start up sound and the apple logo appearing, there never used to be a blank dark screen at that point but now its there for about 60 seconds, then the apple logo appears and normal start up resumes, why is this happening and how can i make it faster again please? I have a 11 inch Mac Book Air that was purchased in mid 2013 and does not have loads of apps or stuff loaded on it.