Cap and rotor too?
What is the mileage and has the head ever been off?
I'm just trying to start the questions rolling in one big shot here, so forgive me for the barrage.
I think that if the cap and rotor weren't changed too, and you now get a hotter spark, you may have some house cleaning to do.
How did the spark plug threads look like on the old plugs?
If you have antiseize or oil and/or other contaminants on the threads, it could take the ignition a little longer (I guess this is relative) to get it's spark sorted out.
I had this same problem too when cold starting but I was burning lots of oil at the time. So the next major question would be, does your motor burn oil currently? If so you may have plugs that are oil caked. Pull em out just to be sure.
okay .... nuff reading, I'm done!