Diagnosis
The bake element is likely weakening but not fully dead. It's heating inefficiently. Temperature sensor could also be off.
How to Fix It
Test the bake element. Unplug range. Access element terminals at back of oven. Measure resistance. Should be around 20-30 ohms. Significantly higher means it's failing. Replace it.
If That Didn't Work
Test the oven temperature sensor resistance at room temperature. Compare to chart in service manual. A faulty sensor gives wrong info to the control.
Heads Up
A slow-heating oven wastes a lot of energy. Do not rely on the preheat indicator. Always verify temp with a standalone oven thermometer.