Surely no oven will reach temperature instantly: the walls of the oven and the air in will be cold, and require heating up. Most ovens will be heating at full blast until their internal termostat tells them that the temperature of the air (or whatever it is they measure) has reached the desired temperature. Once this has happened they will stop heating, just switching on and off as required to maintain the temperature (if you have a gas oven you can see the flames reduce in height dramatically when it reaches temperature).
As bednobs said; logically, this suggests that the oven will heat up at full speed, then turn off when it reaches the desired temperature, only coming back on when the temperature starts to drop.
Fan ovens circulate the air, so they can cook food at a lower temperature and/or more quickly apparently (the food will be in contact with more hot air per second, so more heat will be transferred). I guess this means they will be slightly quicker at heating up, since the hot air will be moved around the interior of the oven, but obviously it will still take time to heat the cold oven walls, etc.