Takes forever for my oven to preheat to 400*. 25 minutes or so. Oven is brand new.

Try checking the heating elements on the top might have come loose

It gas not electric. There is NO hidden element. It a bad sensor.

If it's a gas range and you can see the ignitor glow, but there is no flame within 90 seconds after you turned the oven on - replace ignitor.

First Choice Appliance, Sacramento, CA

The way the oven works is like so:
1) User sets the desired temperature
2) Power is sent to the ignitor
3) The ignitor is then energized, drawing 3.2 amps of current
4) When the 3.2 amps is drawn, the gas safety valve is allowed to open and gas flows into the burner assembly in the oven.
5) This gas is then ignited, causing the oven to heat.
6) As the oven heats up, the oven sensor, which is the metal rod located at the top on the rear oven wall, converts the heat into an electrical signal, relaying this information to the main control which in turns displaying the temperature.
(In models with a dial thermostat, there won't be a sensor, though a metal wire with a bulbous end will be present, which is actually connected to the thermostat.)

If you need further help, I’m available over the phone at https://www.6ya.com/expert/jason_4570d45317834dd3

YOUY NEED A NEW IGNITER

check to see if both broil and bake elements/gas works

http://www.applianceaid.com/nuker.html
click the link  Oven temp sensors sense the temperature is the oven cavity and the temp sensor changes resistance, this change in ohm resistance tells the oven control what temperature the oven cavity at and tells the control to shut off the element or turn it back on. The most common temp sensors used today ( *not all ) are approx 1000-1100 ohm resistance at room temperature ( 70�F )
Some ohm readings and there temperature:
Degrees F    -   Resistance
            100 - 1143 ohms
             200 - 1350 ohms
             300 - 1553 ohms
             350 - 1654 ohms
             400 - 1753 ohms
            500 - 1949 ohms


click this link
http://www.applianceaid.com/elecrange.html

I would be suspicious of the thermostat. It or the sensing element may have failed.

most ovens take 15-25 min to preheat.

Cold Stove, you may not be able to. The issue here is that oven manufactures put on their units a fast preheat. However it isn't possible to preheat an oven for 7 to 9 minutes and bake short time bake items properly. Also the oven sensor will respond faster than the absorbation rate of the sheet metal in the oven liner. To preheat an oven properly takes 15 to 20 minutes before putting in the food. I see this will a lot of current ovens. Sounds like a normal format of the control.