I am having the same problem and restarting does not help. You should note that the 3 second delay only occurs when you start a new tab from a state of having no tabs. i.e. opening the 3rd tab seems smooth because safari has already drawn the tab area. Also this seems to be a problem when working in full screen mode. Try to see if you get the same problem under these conditions.
The solution for me is to Show Tab Bar from the View menu. This keeps the tab bar open even with no tabs (or a single tab open.) This way Safari doesn't hav to draw the tab bar upon opening a new 2nd tab. The 3 second delay is due to Safari drawing the tab bar when working in full screen mode.