Question
The temperature of my resistive heater is slow to stabilize and is not within specification.
Answer
This is probably due to the fact that the setpoint temperature is close to the ambient temperature.
Set the temperature at least 10°C above ambient when using a resistive heater. A resistive heater is unable to precisely maintain temperatures near ambient because once the temperature overshoots the setpoint, the controller turns off and relies on ambient temperature to cool the load. If setting the temperature 10°C or more above ambient is not possible, then choose a thermoelectric controller, which can alternately heat and cool the load to maintain a more precise setpoint temperature.
More information about resistive heaters and resistive heater control can be found in Application Note AN-TC10: Temperature Controller Basics.