Even on a modest project, with my respectable computer, it takes MINUTES to insert a few seconds worth of empty frames into the project.

I usually take care to not split any audio or video assets.  I just insert frames between actions so I can add something else into the middle of my scene.

I could almost do it faster by hand sometimes.

I am sure if someone takes the time to look into the algorithm used to do this, they'll have a "face palm" moment where they find a loop was nested very inefficiently or something like that.

The only good thing is it has worked reliably.  But after 5 minutes you start to wonder.