It's been well documented that people judge the page they are viewing quickly before making a judgment. In his short article Powers of 10: Time Scales in User Experience, Jakob Nielson explains why it's important to understand the purpose of your design in order to optimize the usability.
Jakob Nielsen, Ph.D., is a User Advocate and principal of the Nielsen Norman Group which he co-founded with Dr. Donald A. Norman (former VP of research at Apple Computer). Before starting NNG in 1998 he was a Sun Microsystems Distinguished Engineer. More of his work may be found on his site Useit.com.