Disabling attention detection option will reduce the time Face ID needs to confirm your identity before unlocking iPhone X. Follow below steps to disable Face ID attention detection and turn on attention-aware feature.
Step 1: Open Settings > Face ID & Passcode. Enter your passcode to proceed.
Step 2: Scroll down to ATTENTION section.
Step 3: Toggle Require Attention for Face ID switch to Off position.
Step 4: Click OK on the confirmation screen.
Step 5: Toggle Attention-Aware Features switch to ON position.