That's an interesting point! No we haven't really - for now, I asked a bunch of friends to send me large CSVs - I loaded them on Google Sheets to see how it performs (usually terribly - crashes my browser tab or it's unbearably slow). Then I try the CSV in Pretzel with some filters and pivots and if I don't feel any annoyance with speed, it's a success in my book for now.

We'll definitely need to figure out (a) what's the right thing to measure, and (b) an automated testing/CI to check for regressions and do perf testing