Performance issues

Fast download time continues to be the most important design criterion for web pages.
Response time of 0.1 second is about the limit for feeling that the system is reacting.
Response time of about 1 second helps flow of thought remain uninterrupted. This would support navigating freely through an information space from page to page.
10 seconds is about the limit for keeping the user's attention focused on the dialog. Longer delays encourages turning to other tasks.
Bailout from web pages is predictable based on page size in bytes. Study showed 33K page size = 10 percent bailout; 40K page size = 25 percent bailout.

Reference:
Nielsen, Designing web usability, 2000.