Seems to be taking a long time to cook

Question

Hi we have just been given a whirlpool electric fan oven but have not got the instructions. we cooked something in the oven which to me knowledge usally takes about 20-25 but when cooking in this oven they took about 45minutes. i believe this cooker is probably about 8-9 years old. not sure how to pre-heat the oven do you just put it on the fan symbol for about 10 minutes at the temperature you want to cook at or is there a knack to it

thank you