What's the difference between cumulative and annualised performance rates when evaluating ETFs?

Cumulative means the total return over the period which the ETF is assessed for. Annualised means the annual return for the ETF over the period it is being assessed for.

Taking VOO as an example, as at 31/12/2020:

Cumulative Returns for the 5Y period is 102.80%, which means if you had invested from 31/12/2015 to 31/12/2020, your returns from VOO would've been 102.80%

Annualised Returns for the same period is 15.19%. This means that if averaged out over the last 5 years, each year would have returned on average, 15.19%.

Personally, I feel annualised returns are more useful in comparison, but either method should give you the same "better ETF" when comparing over the same time period for the performance for two ETFs.