I’m using a Raspberry Pi 5 as a Wi‑Fi bridge (AP for my laptop + STA to dorm Wi‑Fi). I expected laptop speeds to approach the Pi’s WAN speed, but there’s a ~40 Mbps gap I can’t explain.
My setup
- Raspberry Pi 5 with Debian trixie, kernel 6.12.47+rpt-rpi-2712
- wlan1 (Edimax EW‑7811UTC) connects to dorm Wi‑Fi
- wlan0 (internal) hosts AP
- NAT/forwarding enabled (NetworkManager ipv4.method=shared, ipv4.ip_forward=1, nftables masquerade)
Commands I used to set up the AP + NAT
- AP (wlan0), 5 GHz, 80 MHz, NAT/shared
sudo nmcli con add type wifi ifname wlan0 con-name Hotspot autoconnect yes ssid PiAP
sudo nmcli con mod Hotspot 802-11-wireless.mode ap
802-11-wireless.band a
802-11-wireless.channel 36
802-11-wireless.channel-width 80
802-11-wireless-security.key-mgmt wpa-psk
802-11-wireless-security.psk "mypassword"
ipv4.method shared
ipv6.method disabled
Key link rates
- Pi → dorm AP (wlan1): 5 GHz, 40 MHz, 200 Mbps PHY
- Laptop → Pi AP (wlan0): 80 MHz, 433 Mbps PHY
Local hop isn’t the bottleneck because iperf gives
- iperf3 laptop ↔ Pi over Wi-Fi: ~252 Mbps
Pi WAN is stable
- Pi → Internet (Cloudflare 50 MB test):
~148 Mbps avg (5 runs)
But laptop via Pi is capped
- Laptop → Internet via Pi (same Cloudflare test):
~107 Mbps avg (5 runs)
So: local hop can do ~250 Mbps; WAN is ~148 Mbps; yet laptop tops out ~107 Mbps. That’s a ~40 Mbps loss.
Question: Where is the ~40 Mbps disappearing? Is this expected overhead for:
- Wi‑Fi AP+STA bridging on a Pi?
- NAT/iptables/nftables?
- TCP over the WAN path with extra Wi‑Fi hop?
- Something like queueing/buffering in NetworkManager’s “shared” mode?
