I can monitor the same thing with my Laptop. It can track the opening of the secondaries and plot it versus rpm, load, tps, anything the PCM sees... It's part of the enhanced manufacturers data. I have tons of comparison charts testing different things. It's addicting...

The bog is due to lack of intake velocity. You are basically starving the engine before the secondaries open the short runners.
I sing the praises of pinned secondaries. This winter I'll even be testing no secondaries.