Stuck at an average of ~40 sec Intermediate, ~120 sec Expert -- how best to improve from here?

Although I'm sure you know this, you didn't mention it in your question.

If you have the number of flags placed around a number equal to that number, then clicking with both left+right mouse will clear all other squares around that number as well.

This is especially useful when there is a sparse area with a lot of 1s, because you can clear a ton of space per click.

As far as I know, no "serious" minesweepers use the question marks; it's generally an option that is disabled with extreme prejudice. And even the marker flags (right-click) are used sparingly so as to not waste too much time. You only use enough to be able to leverage them for the dual-button click & propagate opening a large area.

Here is a page that lists some of these more advanced tips at the bottom, more along the lines of what I'm looking for, but I'm wondering which of these really makes a difference in going from 'meh' pretty fast, to 'holy crap how is that even possible' fast:

http://www.minesweeper.info/wiki/Strategy

e.g. should I work on "efficiency" for a while, instead of raw speed, to see a significant improvement?

I'm hoping someone has personal experience like, 'oh, I remember when this one thing dawned on me and I beat all my top scores by a huge margin'. Because that actually happened to me when I saw this vid:

(which needs to be watched in slo-mo to appreciate)

after which I minimized my flagging and actively tried to take "leaps" into squares behind the front lines instead, where I'd normally just pick at the "surface" squares so to speak, around the edges of the open areas. I dunno how else to describe it