It may be that your thermostat isn't closing or isn't closing fully. The thermostat is designed to include the radiator in the path that circulating coolant follows only when it reaches a specified temperature. This will typically be around 88 degrees Celsius and most vehicles are designed to operate at an optimum temperature (typically around 90 degrees Celsius).
If your thermostat doesn't close properly, the radiator will cause the coolant temperature to drop below the optimum running temperature which can cause poor running.
Another reason for this behaviour may be that there is an air lock / air bubble trapped somewhere within the system. When filling or topping up the coolant it is important the seat the HVAC controls for full heat in the cabin to prevent air getting trapped in the heater matrix. It's also important to "burp" the system which is usually done by running the engine briefly with the coolant cap removed and squeezing the bottom hose (different vehicles have a different procedure, some have bleed screws).
Finally, it may be that you simply don't have a strong enough concentration of anti-freeze in the system and the coolant is partially icing up in the system.
Out of all scenarios, I'd think that the thermostat the most likely. Draining the system, replacing the thermostat and filling with new, correctly mixed coolant, would solve all three scenarios.
NOTE: It is possible to test the operation of the thermostat by submerging it in boiling water in a clear container (that can withstand the temperature) and watching to see that it opens cleanly. Removing it from the water you should be able to watch it close again. If it doesn't move, it's faulty.
