Why does my heat take so long to heat up and not come out fast?

What is your question?

I replaced the heater core and the air coming from the front vents take way too long to heat up. The air in the rear passenger vents never heat up. Its not my blower motor because the air condition works great. The air blows out really fast and cold, but as soon as I turn the temperature up to 90 degrees the heat kicks in but the air coming out deminishes to almost nothing coming out. I can hear the blower motor running but heat not coming out nearly as fast as AC?