There is a gap between the price you see and the price you actually get, and it has a name. Slippage is one of those costs that slowly bleeds a trading account if you never learn to manage it properly.
The basic mechanics are simple enough. When you submit a market order to buy, you get filled at whatever asks are available. If your order is larger than the best ask, the remainder fills at worse prices. The total difference between your expected price and your actual average fill price is your slippage.
Estimating slippage before you trade requires looking at order book depth. If you want to buy 10 ETH and the order book shows only 3 ETH at the best ask, you know you are going to eat into higher price levels. Most exchanges now provide order book snapshots via API, and some trading tools pre-calculate estimated slippage for a given order size.
Execution optimization starts with choosing the right order type. Limit orders give you price certainty but not execution certainty. Market orders give you execution certainty but not price certainty. The middle ground is using limit orders slightly worse than the current market price, which increases your fill probability while capping your slippage.
Time-weighted average price (TWAP) strategies break a large order into smaller chunks executed over a period of time. If you need to buy a large position, spreading the execution over an hour rather than filling it all at once can significantly reduce market impact. Several crypto exchanges now offer built-in TWAP execution tools.
Another approach is volume-weighted execution, where you increase order size during periods of higher volume and decrease it during quiet times. The idea is to trade when the market can absorb your order size more easily, which reduces your footprint.
The venue matters too. The same token can have wildly different liquidity profiles across exchanges. Before placing a large order, compare order book depth across your available venues. Sometimes splitting an order across two or three exchanges gets you a better average fill than placing it all on one.
For most retail traders, the simplest optimization is just switching from market orders to limit orders and being patient. The urgency premium you pay for instant execution is almost never worth it outside of genuine emergency situations.