It depends on whether you divide the larger by the smaller, or vice versa. Say X went from 100 to 50, while Y went from 100 to 100. It can be seen as 50% reduct

It depends on whether you divide the larger by the smaller, or vice versa. Say X went from 100 to 50, while Y went from 100 to 100. It can be seen as 50% reduction in X's value or 100% underperformance vs Y. The 100% number is kind of more useful because it is the extra return on the decision to switch to Y back then.