diff --git a/deploy/files/etc/sysctl.d/99-left4me.conf b/deploy/files/etc/sysctl.d/99-left4me.conf
new file mode 100644
index 0000000..9212bbb
--- /dev/null
+++ b/deploy/files/etc/sysctl.d/99-left4me.conf
@@ -0,0 +1,21 @@
+# Host-side perf baseline for left4me — see
+# docs/superpowers/specs/2026-05-09-l4d2-server-host-perf-baseline-design.md
+#
+# UDP socket buffers: distro defaults of ~128 KiB are too small for sustained
+# Source-engine UDP across multiple instances. 8 MiB matches the standard
+# 1 Gbit recommendation; rmem_default/wmem_default protect sockets that don't
+# explicitly enlarge their buffers.
+net.core.rmem_max = 8388608
+net.core.wmem_max = 8388608
+net.core.rmem_default = 524288
+net.core.wmem_default = 524288
+
+# Kernel softirq UDP path: the per-CPU backlog queue starts dropping packets
+# at the default 1000 under multi-instance burst; 5000 absorbs realistic peaks.
+# netdev_budget = 600 gives softirq more drain headroom per pass.
+net.core.netdev_max_backlog = 5000
+net.core.netdev_budget = 600
+
+# Latency-sensitive default: avoid swap unless the box is really under
+# pressure. Harmless on swapless hosts.
+vm.swappiness = 10
diff --git a/deploy/tests/test_deploy_artifacts.py b/deploy/tests/test_deploy_artifacts.py
index 059b3d3..10efe4d 100644
--- a/deploy/tests/test_deploy_artifacts.py
+++ b/deploy/tests/test_deploy_artifacts.py
@@ -11,6 +11,7 @@ WEB_UNIT = DEPLOY / "files/usr/local/lib/systemd/system/left4me-web.service"
 SERVER_UNIT = DEPLOY / "files/usr/local/lib/systemd/system/left4me-server@.service"
 GAME_SLICE = DEPLOY / "files/usr/local/lib/systemd/system/l4d2-game.slice"
 BUILD_SLICE = DEPLOY / "files/usr/local/lib/systemd/system/l4d2-build.slice"
+SYSCTL_CONF = DEPLOY / "files/etc/sysctl.d/99-left4me.conf"
 GLOBAL_REFRESH_SERVICE = DEPLOY / "files/usr/local/lib/systemd/system/left4me-refresh-global-overlays.service"
 GLOBAL_REFRESH_TIMER = DEPLOY / "files/usr/local/lib/systemd/system/left4me-refresh-global-overlays.timer"
 SANDBOX_UNIT_DIR = DEPLOY / "files/usr/local/lib/systemd/system"
@@ -128,6 +129,21 @@ def test_l4d2_build_slice_exists_with_low_weights():
     assert "IOWeight=10" in text
 
 
+def test_sysctl_conf_present_with_perf_settings():
+    assert SYSCTL_CONF.is_file()
+    text = SYSCTL_CONF.read_text()
+    for line in (
+        "net.core.rmem_max = 8388608",
+        "net.core.wmem_max = 8388608",
+        "net.core.rmem_default = 524288",
+        "net.core.wmem_default = 524288",
+        "net.core.netdev_max_backlog = 5000",
+        "net.core.netdev_budget = 600",
+        "vm.swappiness = 10",
+    ):
+        assert line in text, f"missing {line!r} in 99-left4me.conf"
+
+
 def _fake_command(tmp_path, command_name):
     marker = tmp_path / f"{command_name}.args"
     command = tmp_path / command_name