Hardened slotted/dippled rotors question

[losh1971]

So I recently fitted hardened rotors and new HPX pads but the issue is I am getting some fade on steep declines, if am on the brakes a lot. This used to occur but no where near as much before and normally would be if the trailer was on. It's like the pads are needing a slightly softer rotor to bite in better. Is this even a thing that rotors can be too hard and affect braking?

 

[Sabbath']

How old is the brake fluid?

 

[losh1971]

Brand new fluid I had to replace it because I had to remove a caliper when I fitted the new pads.