1998 Oldsmobile Aurora hard start

i can crank my car in a flash ,first time in the morning it just turns over once, however after driving it enough to get it hot then i shut it down ,after about 15 minutes down time it turns over about 5 to 10 seconds before it will start , once it starts it runs fine . what could this be .thanks this is a consistant problem. i have to burn premium fuel in this car.