Every card authorisation runs through a fraud engine before your bank replies *approve* or *decline*, and it has almost no time to think. Production systems score a transaction in roughly 10–50 milliseconds, fast enough to sit inside the live authorisation round-trip without you noticing a pause. In that window the engine reads hundreds of features: the amount, the merchant category, the time of day, the device fingerprint, the IP, whether the location is plausible given your last transaction, and how this spend compares to your own history.