Adl6010 - low power pulse performance

I am experiencing poor leading edge pulse performance with an adl6010 detector (mounted on my own evaluation board design)

For pulse powers less than -15dBm, there is a delay between RF leading edge and the detected video output voltage (>20ns), however rise time is good, less than 10 ns. This delay is power dependent, increasing for lower powers (up to 80ns!).

Is this to be expected?. All other aspects of the performance appears to be as expected (frequency flatness, power in-voltage out transfer function, rise time, fall time etc.) which leads me to believe that my test pcb and build is sound.