I pretested the necessary time-frame for repeatable results. For whatever reason, I'm not seeing anything similar to the graphs shown above.

I should mention though, that there are two types of temperature courses in my testing procedure: The first run starts with the lowest fan speeds and the water warms up from ambient. I pre-warm the water a little bit, even though it's probably not necessary. So i start the heater, shut it off again after a few minutes, start the logging software and start the heater again. This is a temperature log picked out and just plotted quickly in Excel, showing the entire 35 minutes of a test-run:




After that, the water never completely cools down between test runs, I unplug the heater, adjust fan speeds to the next higher step and then start the heating again. I guess I could also just leave the heater running. Anyway, this means that for all but the first test run, the water is cooling down from what it was at lower fan speeds to a new equilibrium at higher fan speeds. This is such a curve, also picked out of the logs. It shows the change from 400rpm to 800rpm:



So, as you can see, I really don't see this gradual temperature change over a long period of time. The temps seem to reach a certain level after just a few minutes and then just stick to that.

I'm pretty sure I posted the results from 35 min vs 60 min test runs in my setup somewhere in these forums. Can't find it anymore, though.


EDIT @Waterlogged: That might be true. Two rads like the RX might outdo or match a Mo-Ra.