Question for the clever ones... Why does a turbo spool faster under higher load at the same rpm?

 

The engine responds slower to the boost pressure because of opposite forces (load) so the "spool" occurs sooner than when under lower loads where the engine would respond and increase in RPM's sooner instead.

It's the same thing effectively as brake boosting.

 

There are two components to turbo "spool", first is steady state boost threshold and the second is transient response. The first is rpm-based and depends on the turbo hardware being used, while the second is time-dependent. Take the example of an STi with a GT35R and a large FMIC, if you mash the gas in 1st gear from a roll it doesn't hit steady state boost by redline because the time it takes to go through first gear is less than the time the turbo needs to get up to speed (and the piping to be pressurized). Repeat this same experiment in 4th and there's simply a lot more time available for the turbo to spool up vs rpm. This is what we call "load".. Hope this makes things clear to you.