If we restrict our attention to the cherry-picked-with-hindsight US stock market, the worst 30-year period would probably be 01929 to 01959, although 01903 to 01933 isn't that great either. The S&P 500 only goes back to 01957, but https://www.econstats.com/eqty/eq_d_na_4.csv shows the DJIA at US$382.01 on 01929-09-05. 30 years later, on 01959-09-04, it had reached US$653.91. That's 1.8% return per year, which is lower than 10%.

But wait! Those are nominal numbers, not inflation-adjusted numbers. And the dollar had lost nearly half its value over those 30 years! According to https://data.bls.gov/cgi-bin/cpicalc.pl?cost1=382.01&year1=1..., US$382.01 in September 01929 would be worth US$646.99 in September 01959. So the inflation-adjusted return was 0.035% per year. At least it's not negative! And that doesn't even include the dividends!

Nowhere close to competitive with insulating your house or bulk-buying food, though.