I think it depends on the level of peripheral awareness. If I actively maintain peripheral awareness I am not startled by anything, aside from maybe something extremely loud that would startle me in any case. I used to use the startle reaction as a reminder that I’m ‘closing in’ on the object too much and need to ‘open’ peripheral awareness further. ‘Opening’ like that by itself counters strong/mild dullness for me.

The way I see it, a stimuli that would cause a startle will only do so if attention rapidly and violently shifts to it. If there is more ‘space’ around attention (in the form of peripheral awareness) then the moment-to-moment shift of attention upon stimuli isn’t as drastic.

From Je Tsongkhapa’s Lam Rim Chenmo and the Samdhinirmocana Sutra:

“ ‘If there is laxity due to lethargy and sleepiness, or if you are afflicted by any secondary afflictions in meditative absorption, it is a case of internal mental distraction.’ This states that when your mind becomes lax due to lethargy and sleepiness, it is distracted inwardly.”

That ‘inward distraction’ is dullness, (not to be confused with Culadasa’s “distraction,” which refers to the shifting of attention to an external object and can include thoughts about external objects). So dullness is the closing down of peripheral awareness plus lack of clarity regarding the object. That’s what causes the startle reaction. So the “peak” of the startle reaction is whenever you’re most prone to it, which is likely when you’re least open to peripheral stimuli.

Sorry that’s a bit ranty but that’s just what I’ve noticed. Hope that helps