Game Lag White Paper › Problems only some players hit

Spawn messages dropped during loading Spawn messages dropped before the client is ready

Cause ID pt-loading-drop · Primary owner Game team (Client development) · Also Game team (Server development)

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Right after you enter a zone, the server sends spawn messages for nearby NPCs, but the client is still loading the map and throws them away.

Why The server sends spawn messages for nearby entities right after processing the entry → Effect The client is still loading and has no message handler yet, so it drops the messages → On screen The server treats them as sent and never resends them. The NPC stays invisible until it leaves view and comes back

Symptoms

Invisible / ghost entities

Factors

Packet loss

Who’s affected

One client on the same PC, Just me

When

Right after login or maintenance, While moving or changing zones

Owner

Primary owner Game team (Client development) · Also Game team (Server development)

Game team action items

Client: send “ready” when loading finishes, or hold packets received during loading and process them afterward. Server: send nearby information only after receiving “ready.”

Ballpark numbers

If two clients load at the same time on the same PC, or the loading one is a background window, they share CPU and disk and processing is throttled, so that client’s loading can take several times longer. The same bug also surfaces when the server gets faster at processing entries.

On the graph

Outliers only · Loading time per client, messages dropped during loading

Where to look

Count and type of messages the client received and dropped during loading and the time loading finished, compared with the time the server sent the spawn messages. Easy to reproduce by loading two clients at once on the same PC or leaving the loading one as a background window

Confirmed if

The server sent the spawn message for the missing NPC, it arrived before loading finished, and the dropped-message count rose at that time. Happens only on the client with the longer loading

Ruled out if

Spawn message arrived after loading finished but the NPC is still invisible: lost baseline snapshot or entity ID reuse mix-up. The server never sent that NPC’s message at all: AOI registration race or a channel/phase difference

Check with

Game server or client logs and metrics

Sources

1. NetworkConfig class (Netcode for GameObjects 2.5) Unity
   SpawnTimeout: messages for objects not yet spawned are held, then dropped if the object isn’t spawned in time
2. Actor Relevancy in Unreal Engine Epic Games
   Actors that are no longer relevant are destroyed on the client and replicated anew when they become relevant again

See also

Same layer: Problems only some players hit

• A lagging player moves in bursts on others’ screens
• Fast-forward on servers that process on arrival
• Per-player input buffer size
• Validation false positives concentrated on one ISP
• One lagging party member and boss mechanics
• A lagging client controls the monster
• Bloated data on one character
• Different channel, instance, or phase
• AOI registration race
• Lost baseline snapshot
• Missed despawn message (ghost entity)
• Spawn data lost in the burst after entering
• Entity ID reuse mix-up
• Fixed UDP port collision
• Sessions keyed by IP or device (bug)
• Multi-client restriction
• Background window throttling
• Simultaneous access to cache or asset files
• Streaming failure from memory or VRAM shortage
• Different display settings
• Client version or data mismatch
• Per-connection send budget and priority
• Entities held back by clock estimate error

Same symptom (Invisible / ghost entities), other layers

• Slow storage delays asset streaming L1 Client game process
• Keepalive default of 2 hours L8 Sockets and protocols
• Slow start after idle L8 Sockets and protocols

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