People almost always describe this the same way. The oven used to be ready by the time the tray was filled, and now it is not, and nothing else has changed. Sometimes there is a second half to the sentence: things brown on one side, or the top of a cake sets before the middle does.
Underneath that description there are only two possibilities, and they are worth separating because they lead to completely different repairs.
Possibility one: it is making less heat
An electric oven element does not always fail cleanly. It can open up in one section, keep glowing along the rest of its length, still feel hot to the hand, and be doing perhaps two-thirds of the work it used to do. Everything looks right and nothing is right.
The same thing happens more quietly with a temperature sensor. A sensor that has drifted a few degrees is not broken in any way you could see. It simply tells the control that the cavity is hotter than it is, and the control obediently stops calling for heat too early. The oven is then perfectly obedient and perfectly wrong.
Possibility two: it is losing heat
A door seal that has gone hard and taken a set will let a thread of heat out along one edge. You can often find it with the back of your hand held an inch from the door, moving slowly around the frame, while the oven is up to temperature. Where the air is warm, the seal has stopped sealing.
Hinges do the same thing more slowly. A hinge that has weakened lets the door sit a fraction proud at the top while still latching cleanly at the bottom, and the gap it opens is small enough to look like nothing and large enough to cost you fifteen or twenty degrees.
How we tell them apart
We put a measured temperature in the cavity and compare it with what the control believes. If the control is asking for 350 and the cavity is sitting at 320 while the element is still working hard, heat is going somewhere. If the element has stopped calling for heat while the cavity is still cool, the sensor is lying. That distinction takes one visit and it decides everything after it.
The third possibility, which is not really a third
There is a version of this complaint where the oven is neither making less heat nor losing it, and it is worth naming so you do not chase the other two for nothing.
If the cooking has changed rather than the oven, the symptoms can be identical. A heavier tray, a fuller oven, a dish going in straight from a cold refrigerator rather than from the counter, or racks loaded so tightly that air cannot move between them will all lengthen cooking times and produce uneven browning. So will a convection fan running slower than it should, which is a genuine fault but a mechanical one rather than a thermal one.
We mention it because it is easy to test at home. Preheat the oven empty, with one rack in the middle, and see whether it reaches temperature in something like the time it used to. If it does, the oven is fine and the loading is the variable.
Before you call
Two things are worth checking yourself, and both are free. Look at the door seal for a section that has flattened or hardened, and feel around the closed door for warm air while the oven is hot. If you find warmth at one corner, say which corner when you ring. It is the single most useful sentence you can give us.