DeGiro vs the S&P 500 or MSCI World: comparing fairly
The index's own return and a fair comparison to it are not the same number, and the gap between them is entirely about timing.
Why the obvious comparison is unfair
The index's quoted return for the year assumes one thing: money in on day one, left alone until the last day. That's almost never how a real account works — deposits arrive through the year, in whatever amounts and on whatever dates suited you at the time. Comparing your account's return against the index's headline number is really comparing your actual, staggered deposits against a hypothetical lump sum that was never actually available to you on 1 January. If the index happened to fall early in the year and recover later, a lump-sum investor eats the whole drop and a gradual one doesn't — and that's true even if both of you are otherwise doing exactly the same thing.
What a fair comparison actually does
Take your own deposits and withdrawals — the real ones, on the real dates — and simulate putting that same money into the index instead, on those same dates, marked to market every day after. That produces a benchmark value series built from your own cash-flow history rather than from an assumption, so the two lines on the chart started from the same money at the same times and can be read against each other honestly.
The comparison this tracker draws on the Overview tab is built exactly this way — your deposits, your dates, your chosen index — rather than as the index's own quoted return sitting next to your account's.
Picking an index to compare against
The S&P 500 is the 500 largest US companies; an all-world tracker adds developed and emerging markets outside the US; the Nasdaq-100 is tilted hard toward large US technology companies and is a noticeably more concentrated bet than either of the other two. None of the three is automatically “correct” to compare against — it depends what you'd have put the money into if not into the positions you actually hold, and a portfolio concentrated in one sector or region is fairly compared against something that shares that concentration, not against whichever index has done best lately.