qbt: tweak

configuration.nix

@@ -132,6 +132,31 @@
       compressor = "zstd";
       supportedFilesystems = [ "f2fs" ];
     };
+
+    # BBR congestion control handles variable-latency VPN connections much
+    # better than CUBIC by probing bandwidth continuously rather than
+    # reacting to packet loss.
+    kernelModules = [ "tcp_bbr" ];
+
+    kernel.sysctl = {
+      # Use BBR + fair queuing for smooth throughput through the WireGuard VPN
+      "net.core.default_qdisc" = "fq";
+      "net.ipv4.tcp_congestion_control" = "bbr";
+
+      # Disable slow-start after idle: prevents TCP from resetting window
+      # size on each burst cycle (the primary cause of the 0 -> 40 MB/s spikes)
+      "net.ipv4.tcp_slow_start_after_idle" = 0;
+
+      # Larger socket buffers to accommodate the VPN bandwidth-delay product
+      # (22ms RTT * target throughput). Current 2.5MB max is too small.
+      "net.core.rmem_max" = 16777216;
+      "net.core.wmem_max" = 16777216;
+      "net.ipv4.tcp_rmem" = "4096 87380 16777216";
+      "net.ipv4.tcp_wmem" = "4096 65536 16777216";
+
+      # Higher backlog for the large number of concurrent torrent connections
+      "net.core.netdev_max_backlog" = 5000;
+    };
   };
 
   environment.etc = {