X-15 derate warning

[GearGrabber18]

I just recently purchased a 2018 579 with X-15 truck has 710k mi, truck is drops about 10mph when pulling hills empty and is getting poor fuel mileage (5mpg) with light loads (15k) I also keep getting a warning for derate in 3hrs in which I let it go into a level 1 derate when it did this the truck didn’t lose power, how I cleared this warning was flip the key back and forth (7x) it’s not showing me any active code or faults. Truck is just lacking power, getting horrible MPG and throwing derate warnings, any advice will be greatly appreciated.