Oven issue: temp sensor or new thermostat?

Oven/Range/Cooktop Repair
1

Brand: GE Cafe
Model Number: CS980S
Main Symptom: The oven is very slow to get to the desired temperature
What happens & when: In the past few weeks, the oven has gone from 30 minutes to get to desired temp to over an hour to get to the desired temp. Plus, the food is cooking slow.

Error Code (if any):
Parts or tests already tried: GE recommended a new Thermostat which I ordered. While looking for installation instructions on YouTube, I ran across installation guides for a Temperature sensor. Questions: how do I determine which part is faulty, the Thermostat or Temperature sensor? And if the thermostat, where can I find guidance on how to install?
Photos / video link:

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It looks like the model provided is a partial model. If the full model number could be provided, it would help with providing the most accurate information possible. This could be an element failure or a temperature control (thermostat only or a control and sensor) failure.