is it just me or do HPS bulbs work better in the winter? I'm guessing it's because HPS bulbs give out slightly more warmth and help the plants grow better in the winter? I find that my LED light just doesn't work very good in the winter and the plants growth is very slow.
The plants like it warm and definitely grow better in warm temperatures, but if it’s too warm they slow down completely.
That said, it may just be the quality of your HPS bulb or your LEDs. To test, you could just measure temperature and humidity with the HPS running, then add a space heater to the room when running the LEDs and get the temperature the same as when you run your HPS. With the temperature the same, the big variable would be the light quality. That space heater may dry the area out more so you may also need a humidifier to duplicate the conditions, but maybe not.
HPS bulbs perform far less effectively after each grow. Don’t just wait until they burn out, replace them after two or three grows.