Mine warms up to operating temp. within ~10 minutes in similar ambient temperatures to yours. When it had a broken thermostat it exhibited behavior similar to yours.
Failed Thermostat
Did you test the new thermostat before installation? I had one that was defective right out of the box. Is it the correct thermostat for the engine? It needs the tophat disc to close off the relief (bypass) port when warmed up.
Does the cabin heater put out insufficient heat also? If the heater core produces good heat then the problem may be the temp gauge sender. Located just forward of the radiator hose outlet in the head.