I have a weird issue I moved into a new house recently and I have some old oven in here I bought a thermometer which is hanged inside and it seems like the temperature is correct however, anything I put inside is not baking in the time it should. For example I have cookies recipe that requires 160 degrees Celsius [=325f] for 10 min, but they are still not fully baked after 17min at 350f or ~22min on 325 [almost done though] Same for a simple frozen pizza. Instructions are 400 degrees for 18-21mins, but it is close to be done after 25+ mins. The cheese at the top is still not fully melted after that period. It may sound weird but the only thing I can think of is the baking sheet. I have: Nordic Ware Natural Aluminum Commercial Baker's Half Sheet, 2 pack, 2-Pack, Silver Maybe this is not the right sheet for baking and I should have ones from a different material and a different color [in my home country the sheets are black] For the cookies I also put them after they were in the freezer. They should be cold while going into the oven, but maybe this is too cold and it contributed to the slow process UPDATE: Hey! So I checked and only the lower element is working. Here is the catch - The company is claiming that this is the baking element and the top element is the broiling, so only one of them will work at the time. Does this make sense to you all? 
The colour of the baking sheets does have some effect, but not as great an effect as what you're describing. My guess would be that one of the heating elements is broken (probably the top one since you mention cheese not melting). The bottom element can get the oven to the correct temperature, but if heat is only radiating from the bottom that will affect how things bake. 