The “time” key that I see in the application/…/rx payload (inside rxInfo) what does it refer to?
The time when the package was received (GPS time of gateway, only set when available). See: https://docs.loraserver.io/lora-app-server/integrate/data/
Please note that by default RX1 is 1 second after transmission. Increasing DEDUPLICATION_DELAY means LoRa Server will wait longer during the de-duplication process, meaning there is less time for scheduling the downlink. You might be careful with this, especially when there is a high latency between the network-server and your gateway. Or you need to increase the RX1 delay so that you have more margin.
Simplified example: by default you have 1000 ms for the downlink. When your gateway has a 300 ms latency to the the network-server, there is 400 ms left for LoRa Server to respond (uplink 300 ms + downlink 300 ms). Now LoRa Server will wait 200 ms for the de-duplication, so you have a margin of 200 ms. So increasing this too much means you’ll never be able to schedule the downlink in time.