Why followers use marketable-limit orders
By default, every follower order MirrorFill places is a marketable-limit order priced 2 ticks through the leader's fill price. You bought ES at 5000.00; a follower's buy goes in as a limit at 5000.50. It behaves like a market order — it crosses the spread and fills right away in a normal book — but it carries a built-in worst case.
What "two ticks through" buys you
A plain market order accepts whatever price it takes to fill. Usually that is fine. In a thin book or a fast move, it can mean filling several ticks beyond the leader's price, with no ceiling at all. Some broker environments also reject bare market orders outright.
The marketable limit keeps the fill-now behavior and adds a cap: the follower will pay up to 2 ticks past the leader's fill and no more. Slippage between the leader's price and the follower's is bounded, by construction, on every entry and every exit.
The honest trade-off
A cap means the order can miss. If the market runs past the limit before the follower fills, the order rests instead of chasing. That is deliberate — chasing at any price is exactly what the cap exists to prevent — and it is handled, not ignored:
- Order expiry, if you set it, cancels a resting follower order after N seconds and writes the missed quantity off rather than re-chasing it at market.
- Reconciliation corrects any remaining drift between what a follower holds and what it should hold, in a quiet window, and it skips accounts that still have an order resting. See how drift gets fixed.
Nothing here is about speed — the order type does not change how fast a copy happens, and we do not quote latency numbers. It is about bounding price, and accepting a defined miss instead of an undefined fill.
Quick answers
Is a marketable limit slower than a market order?
No meaningful difference — both are sent when the leader's fill arrives and both cross the spread. The difference is only what happens to price in a thin or fast market.
What happens if the order never fills?
It rests at its limit. With order expiry set, it is cancelled after your N seconds and the quantity is written off. Reconciliation handles any drift that remains once the order is gone.
Can I switch a follower to plain market orders?
Not in the app today. Marketable-limit with a 2-tick cap is the default for every follower, partly because some environments reject bare market orders.