How to know the progressive state of mind in insight meditation
I believe the speed of letting go of things is one of the indicators from Indriya-bhavana Sutta


  There is the case where, when seeing a form with the eye, there arises
  in a monk what is agreeable, what is disagreeable, what is agreeable &amp;
  disagreeable. He discerns that 'This agreeable thing has arisen in me,
  this disagreeable thing... this agreeable &amp; disagreeable thing has
  arisen in me. And that is compounded, gross, dependently co-arisen.
  But this is peaceful, this is exquisite, i.e., equanimity.' With that,
  the arisen agreeable thing... disagreeable thing... agreeable &amp;
  disagreeable thing ceases, and equanimity takes its stance. Just as a
  man with good eyes, having closed them, might open them; or having
  opened them, might close them, that is how quickly, how rapidly, how
  easily, no matter what it refers to, the arisen agreeable thing...
  disagreeable thing... agreeable &amp; disagreeable thing ceases, and
  equanimity takes its stance.