RTSP re-stream from Nx Server lags several seconds behind live — breaks time-aligned analytics overlay

I'm pulling a camera's RTSP stream from the Nx Server (rtsp://<server>:7001/<deviceId>) into an external analytics pipeline (GStreamer/DLStreamer), then pushing detected object metadata back to Nx via the Analytics Integration REST API (.../deviceAgents/{id}/metadata/object) using each frame's RTCP reference timestamp (add-reference-timestamp-meta / sender_ntp_unix_timestamp_ns). Overlays only render when the pushed timestampMs matches Nx's live frame timeline.

Problem: the frames delivered on the Server's RTSP re-stream are several seconds behind the live edge, so the reference timestamps are stale and the pushed boxes fall outside Nx's live overlay window (objects are stored — they show in notifications — but no bounding box on the Live tab).

Question:
1. How to identify the delay ?
2. How to get timestamp of rtsp stream consumed at nx server and rtsp stream pushed to external analytics by nx server ?