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Subnautica 2 Co-op Desync & Lag Fixes

Short answer

Most Subnautica 2 co-op desync, lag, and 'session falls apart after 1–2 hours' reports come from one of five causes: host upload bandwidth, antivirus blocking the netcode, an overlay (Discord/MSI/RTSS), a VPN on one side, or Wi-Fi packet loss. Fix them in that order.

Symptoms

If any of these match, you're in this guide: creatures rubber-band; doors won't open for the client; a guest's deconstruct doesn't register; everyone freezes for 5–10 s every few minutes; the session crashes after roughly an hour of stable play; one client's FPS tanks when the host enters a Tadpole.

Root causes & fixes

  1. 1

    Host upload bandwidth

    Run a speed test on the host. You want at least 5 Mbps upload per guest. Anything under 3 Mbps upload total and the session will not stay stable with 3 guests. Use wired Ethernet on the host, not Wi-Fi.

  2. 2

    Antivirus / Windows Defender

    Add the Subnautica 2 install folder to your AV exclusions on all machines. Defender's 'Controlled folder access' is a known killer of the multiplayer netcode. After excluding, restart the game.

  3. 3

    Disable overlays

    Turn off Discord in-game overlay, MSI Afterburner / RTSS, Razer Cortex, NVIDIA ShadowPlay overlay, GeForce Experience FPS counter. Test with all overlays off first; re-enable one at a time if needed.

  4. 4

    VPN — turn it off on both sides

    A VPN on either host or client routes Subnautica 2 traffic through an extra hop that often loses packets. Both sides off, or both sides on the same exit, is the rule.

  5. 5

    Wi-Fi → Ethernet

    If anyone is on 2.4 GHz Wi-Fi, switch to 5 GHz minimum, ideally Ethernet. The chassis-bug FPS drop is consistently worse over Wi-Fi due to packet retransmits.

Long-session crash (1–2 hours)

Confirmed by antberry's deep-dive and corroborated on Steam discussions. Mitigations: have the host save and rotate hosts every ~90 minutes; or have everyone restart the game between sessions even if it feels stable. Hotfix 2 (2026-05-23) reduced but did not eliminate this. Fix tracking — see Updates feed.

Tadpole chassis FPS drop

When the host pilots a Tadpole, clients can see FPS halve. Workaround: have the chassis pilot be the player on the strongest connection, or have clients stay at least 200m away from a piloted Tadpole. Tracked as a known bug — see /bugs/known-bugs-master-list-from-game8.

FAQ

Why does Subnautica 2 co-op crash after an hour?

Known long-session memory leak. Hotfix 2 reduced frequency. Rotate hosts and restart clients every ~90 minutes as mitigation.

Does antivirus cause Subnautica 2 multiplayer issues?

Yes. Add the install folder to AV / Windows Defender exclusions on every player's machine.