Everyone always quotes the growth of the S&P500, but isn't pretty much no one 100% invested for their entire retirement in the S&P500? My 401k is in a target date 2055 and my Roth is split between FXAIX (S&P500, 55%), FSPSX (international, 20%), FSMAX (extended market, 15%), FXNAX (bonds, 10%). It's a little conservative but not that conservative.
Fidelity says my Roth 1Y returns are 10.8% compared to S&P 500's 10.3%. It says my 1Y returns on my target date 2055 are 18.0%. Neither of those numbers can be accurate so it's hard to know what to read in to them. If I try to calculate my returns in a very simple way (take current value, subtract contributions from the last 12 months, which can be easily looked up, call that number X, then find the growth rate that takes the account value I had as Nov. 1st last year and compound that at different rates until it produces X as of now - this gives an upper bound on returns, since the returns of the various money deposited throughout the year at random times is treated as not growing at all), I get 1%. And that's 1% before inflation.
I know the S&P500 is 10% YoY over really long time scales, and I also know that number is like +/-15% year to year. But it feels like my fund picks are pretty normal yet they're not worth any more than what I put in to them since I started saving. Because of that, I'd have to have a 30+% savings rate in order to catch up to the "X salary by Y age" rule because the assumptions over the growth rate of the accounts are wildly off in the years since I started investing.