05 Accord idling speed

The cold start idle at 1500 is normal and when the engine operating temperature increases, the IAC would reduce the amount of air getting to the intake manifold.

This process is controlled by the PCM ( engine computer ) and regulation of air is by the Idle Air Control ( IAC) valve. If the Check Engine Lamp is not indicating, the most likely cause would be dirty throttle body and IAC. Getting them cleaned should solve the problem.

The simple way of cleaning the throttle body is with the aid of carb cleaner.
Keep ignition turned off while doing this.
Remove the air intake hoses.
Spray some carb cleaner into the throttle opening and the ports inside it. Allow to soak for a few minutes.
Repeat process and get a rag to wipe the inner barrel of the throttle body. You can manually push the throttle open and the main areas would be where the throttle plate is stopped at throttle fully closed position. Ensure they are clean.

Allow the carb cleaner to dry out and install air intake hoses.
Start engine and allow to warm up and note if idling speed stabilises.
Starting might be difficult due to the presence of carb cleaner and might stutter after it starts.