Generally the default fund in a decent 401(k) (employer managed but largely employee funded retirement plan) will be a "target date retirement fund" that contains a mix of equities and bonds according to the expected risk tolerance of someone at that age, and generally the equity component is also split between US and foreign equities.

e.g. Fidelity's fund for people planning to retire around 2050 https://fundresearch.fidelity.com/mutual-funds/summary/31579...

3% less annual returns than the S&P500 over 10 years and the entire fund lifetime. That's a considerable difference.

target date funds always confused me, b/c they don't account for the rest of my portfolio.

My IRA/401(k) actually has close to 100% cash/bonds to minimize annual taxable impact and the rest of my portfolio is equities.