An annual charge of a fraction of a percent sounds immaterial next to the swings a portfolio experiences in a single week. Over a long horizon it behaves very differently, and the reason is structural.
A charge is levied on the balance, not the gain
Most investment charges are calculated as a percentage of assets held, so they are deducted whether the portfolio rose, fell or did nothing.
That makes the charge a certainty while the return is not, which already distinguishes it from every other factor in the outcome.
It also means the amount paid grows as the portfolio grows, so the cost in money terms rises exactly as the balance becomes significant.
The loss is the fee plus its foregone growth
Money removed this year is not merely gone. It would otherwise have remained invested and earned returns for every remaining year of the horizon.
Each annual deduction therefore removes a small amount and a much larger stream of future growth attached to it, and those streams accumulate.
This is why the gap between two portfolios differing only in charges widens rather than staying proportional, and why it widens fastest in the later years.
Costs appear in more places than the headline
The stated management charge is one component. Funds also incur trading costs inside the portfolio, which reduce returns without appearing as a separate line.
Platform fees, advice fees and custody charges may each be modest, and they stack, so the total borne by the investor can be several times the headline figure.
Comparing products on the management charge alone therefore measures one input rather than the cost of holding the investment.
Turnover creates costs that are hard to see
Every trade inside a fund incurs spread and commission, and in taxable accounts it can also crystallise a liability that would otherwise have been deferred.
A strategy trading frequently must therefore outperform by enough to cover those costs before it delivers anything to the investor.
Low-turnover approaches avoid most of this simply by doing less, which is a large part of why passive vehicles cost what they do.
Costs are the one variable that is known in advance
Future returns cannot be known, and forecasts of them have a poor record. Charges are published, contractual and knowable before any money is committed.
That asymmetry is what makes them worth attention out of proportion to their apparent size, because reducing them is a change with a certain effect.
The relevant question for any charge is what it is buying, since a higher cost can be entirely reasonable where it purchases something the alternative does not provide.