In Microsoft Defender for Office 365 Safe Attachments, Dynamic Delivery is specifically designed to shorten perceived delivery time while keeping full detonation scanning in place. Microsoft explains that with Dynamic Delivery, the service "delivers the message body right away and replaces attachments with a placeholder while scanning"; once the sandbox verdict is clean, the original file is reinserted automatically
. If malware is found, the attachment is blocked/removed and the message is handled per policy (for example, quarantined). This option meets both requirements in the question: (1) users get messages quickly (no long wait for full message delivery) and (2) security isn't reduced because attachments are still detonated before users can open them.
By contrast, Block delays or stops delivery when malicious; Replace swaps detected-malicious attachments with a .txt summary but doesn't reduce the initial delivery time for clean items; Monitor only logs without enforcing protection. Therefore, to reduce delivery latency without compromising protection and still block malware, configure Dynamic Delivery in your Safe Attachments policy.