I've done the calculation on the S&P 500 for the timespan 01/1900 - 01/2021 with dividends reinvested, and the return would have been slightly below 10%.
This means, had a low cost index fund on this index been available in 1900 (the first one was actually introduced to the public in 1975 with a relatively high yearly expense ratio of around 0.5%) with almost no fees, a net worth passed from generation to generation would have compounded at almot 10% per year, before inflation.
This yearly increase in net worth is without a doubt impressive, but of course not spectacular, however, most professional money managers fail to generate alpha, as will most private investors.
American citizens have access to the lowest cost index funds available on the S&P 500, with a yearly expense ratio of only 0.03% (Vanguard & Blackrock offer them) ! Add to this that discount brokerages offer all ETF purchases for free, the total cost of ownership (brokerage fees + fund fees) can be reduced to around 0.05% per year.
That is, if you invested for a full 50 years in the broad american stock market, it'd only cost you 2.5% in total expenses. It's probably the best option for many folks if they have at least 25 years or more before them. Ideal would be to save as soon as one can, without trying to time the market. Ed Thorp, maybe one of the smartest folks around, suggests so, as of course does Warren Buffett.
What may be a sensible idea aka "middle road" for investors wanting to go the route "active" aka place bets on the future direction of stocks, currencies etc ?
20% per year is probably too ambitious, but 15% per year may be in the cards with a lesser edge. 
