> The bottom line for me was that fetching a couple thousand cells from Sheets as a JS array and then just processing them in JS without touching any of the Apps interfaces turned out to be a couple of orders of magnitude faster than trying to figure out exactly which cells I needed and fetching then individually, even if I only really needed a couple dozen in the end.

Weird, when I had worked with Google Apps Script, it had been significantly faster than working with the APIs by an order of magnitude or so. (although the feeback loop is still painfully slow)