Figure 1 shows the cumulative performance of USMV versus SPY since 2013 on the top graph and the volatility forecast of SPY over the same period. The dark blue line shows the cumulative relative performance, the light blue line shows the returns of SPY, the green and red lines are the upper and lower bounds, respectively, of a 95% confidence interval. This type of risk model is commonly referred to as Value-at-Risk. On average, 5% of returns should fall outside of this interval. In this case, 5.6% are outside of these bounds, which indicates the model fits the data well. The yellow area shows USMV outperformed SPY recently when volatility increased.
Figure 1 indicates that USMV has outperformed SPY during the recent period of volatility but underperformed when volatility was low. Intuitively, this makes sense: a low risk strategy should outperform the overall market when conditions deteriorate.