You click buy on Nasdaq at 19,300. The confirmation comes back at 19,306. Six points, roughly sixty dollars per contract on a typical position, gone before the trade even starts moving. That gap between the price you clicked and the price you got is slippage, and it is not a platform malfunction. It is the market moving between your click and the fill, and every trader who trades fast markets pays it eventually.
Why fills miss the price you saw
An order takes a small but real amount of time to travel from your click to the exchange or liquidity provider and back, and price can move in that window, especially in a fast market. During calm conditions the gap is often nothing, a fraction of a pip on EURUSD. During a news release, an opening bell or a thin overnight hour, the gap widens because there are fewer resting orders at each price level to absorb your trade, so the fill has to reach further to find enough size.
How much slippage is normal, by instrument
EURUSD in London or New York hours typically slips less than a pip on a market order under normal conditions. Gold, trading in bigger dollar increments and reacting harder to headlines, commonly slips 20 to 50 cents on a fast move and can slip several dollars around a major data release. Nasdaq and the S&P 500 slip a few points in calm conditions and considerably more in the first sixty seconds after the New York open or a surprise headline. Crypto is the least forgiving: Bitcoin can slip 0.1 to 0.3 percent of price even in normal trading, and far more during a sharp move, because liquidity thins out faster than in the older, deeper forex and index markets.
The moments that make it worse
Three conditions reliably widen slippage: the seconds around a scheduled release like nonfarm payrolls or a rate decision, the first minute of a session open when liquidity is still arriving, and any thin overnight hour when few participants are quoting. Trading a stop order through any of these is how a five-point stop turns into a fifteen-point loss on the fill, because a stop order becomes a market order the instant it triggers, and it takes whatever price is available, not the price on the ticket.
Habits that actually reduce it
A limit order controls your fill price exactly but may not fill at all if price runs past it, which is a fair trade in fast conditions: a missed trade costs nothing, a bad fill costs real money. Avoiding new market entries in the sixty seconds around a major release, and simply waiting for the initial spike to pass, sidesteps the worst of the gap between click and fill. Reviewing your own execution reports occasionally, comparing intended price to actual fill across recent trades, tells you honestly how much your own instrument and habits are costing you, which is more useful than any general rule of thumb.
Position sizing itself is a quiet slippage defense. A trader risking 1 percent of a $10,000 account, $100, against a EURUSD stop 20 pips away is sized around an assumption of a clean fill near that stop. If the fill lands 3 pips worse during a fast move, the real loss becomes $115 instead of $100, a manageable difference. A trader risking 5 percent on the same trade sees that same 3 pip miss turn a $500 planned loss into $575, and the gap compounds every time the market gets fast. Conservative sizing does not eliminate slippage, but it keeps an ordinary bad fill from turning into a damaging one.