Flushed radiator refilled then van runs sluggish

The heater in my 98 Pontiac Transport was not blowing hot air out it was only warm. We were going to add anti-freeze to the radiator thinking it might be low. And when we opened the cap there was alot of brown looking rust built up on it, so we decided to flush the system out. We empited the radiator and then added the anti-freeze. Then when we took it to the store it was acting sluggish when you tried to speed up what could cause this.