Higher load = higher combustion temperatures = higher valve temps = higher valve spring temps/oil temps =

Yes, the engine could go into valve float much sooner pulling a heavy load up a long grade vs. winging it in your driveway out of gear.

There are many more factors influencing physics than speed, acceleration and mass.

Anybody that has ever had a bad set of valve springs for a while knows that they get worse as the engine warms up.