Immediacy: What You Pay For Speed

Every trader faces the same fork: cross the spread and trade right now with a market order, or place a limit order and wait for someone else to trade with you. The first path is certain but costs the spread (or worse, if size runs past the top of book). The second path is potentially free — a passive limit order can earn the spread instead of paying it — but comes with no guarantee of filling, and the price might move away before it does. Immediacy is the name for what you're paying when you choose the first path: the premium for certainty of execution, right now, over the uncertain but cheaper alternative of waiting.

Where the cost comes from

A limit order sitting on the book is a standing offer to trade, and someone providing that offer bears the risk that the market moves against them before it fills — the classic adverse-selection risk market makers price into the spread. A market order taker doesn't bear that risk; instead, they compensate the liquidity provider for bearing it, by paying the spread. That payment is the price of immediacy: it buys certainty (the order fills now, at a known worst-case price) in exchange for giving up the chance of a better price that patience might have won.

The cost of immediacy isn't fixed — it scales with how urgently you need to trade and how much size you need. A single small market order pays roughly half the spread relative to the midpoint. A large order needing to trade now pays the spread and then keeps paying as it consumes deeper, worse-priced levels of the book — urgency compounds with size because you're not just crossing the spread once, you're sweeping through everything available at each successively worse price to get filled immediately.

Worked example

A stock quotes $50.00 bid / $50.04 ask, with 1,000 shares at each level. A trader needing to buy 1,000 shares immediately crosses the spread with a market order and pays $50.04 — a $0.02 premium over the $50.02 midpoint, or $20 total, purely for not waiting. A second trader with the same need instead posts a limit order at $50.01, inside the spread, and waits: if it fills, they pay $0.01 less than midpoint instead of $0.02 more — a $30 saving on 1,000 shares, roughly $0.03 per share better than the urgent trade. But the limit order might not fill at all if the price moves up before anyone trades with it, in which case the trader either chases the market (paying immediacy anyway, now at a worse price) or misses the trade entirely. The $30 saving is the reward for accepting that uncertainty; the $20 cost is the price of removing it.

What this means in practice

Optimal execution algorithms exist precisely to manage this trade-off: rather than picking one extreme, they blend passive resting orders (cheap, uncertain) with occasional aggressive fills (expensive, certain) to balance execution cost against the risk of not completing the order in time. The right mix depends on how much the trader's information is likely to decay — a trader with a short-lived signal has to pay more for immediacy because waiting risks the opportunity disappearing entirely, while a trader with no urgency can lean almost fully on patient limit orders.