The liquid in your loop and the copper/brass in your radiators should provide a large amount of thermal mass. This means every joule of energy given up from the CPU and GPU will contribute to less temperature rise (compared to a smaller thermal mass like less liquid).
What I'm getting at here is that it should take minutes, maybe even tens of minutes to heat soak your liquid and radiators up to, say, 50°. That's a 25 degree increase from ambient. Components will then have a delta over liquid temp - GPU is usually only a few degrees but CPU can easily be 20-30° over liquid temperature, under load. Meaning 50° water would give maybe 70-80° CPU temp. If you're finding temperatures rise to maximum faster that, something is up. Regardless of overall cooling capacity (defined by radiator area, fan speed and airflow/temperature), there should be some inertia on components temps.
Also, new radiators need cleaning. Some people have experienced some brands coming fairly clean out of the factory. I wouldn't take the chance, there's always some kind of crud to flush out
