delay booting

i want to share with you my experience.one day ago i installed mavericks to my imac 27 late 2012.i am using by the way an apple usb drive,when i press the power on button imac remains in black screen for 25 sec, iam hearing a little noise from the usb drive and afterthat it starts booting.this one began after mavericks installation.it doesnt start at once as it used to do before mavericks.is it the usb drive?i tried reset PRAM but it didnt help.any ideas?