Fees are the single most reliable predictor of your investment performance.
Why? Because your real return (what you actually get) is calculated after subtracting all fees.
For example, if you earn a return of 10% but pay fees of 3% you really only earned 7%.
Although a fee difference of 0,5% or even 1% pa between different investment portfolios may sound small, the long-term impact of excessive fees may well be the decider between retiring comfortably or not.