Howto configure linux router to allow clients to use maximum bandwidth?

Question

The problem is that I'm always limited on the final client machine to about 1/3 (one third,about 200/300kb) of the internet hotspot bandwidth when testing download speed with wget.

I have configured a Debian 7 linux router (named 'server') with 2 wifi adapters:

• wlan0 -> old belkin wifi card on pci at 33mhz(BCM4306)(54Mb/s) using Hostap to make it a hotspot
• wlan1 -> new RTL8192CU on usb (54Mb/s) Connected to another hotspot (netbox, that has Internet connexion, but no way to change any config on it).

so the route throught internet is

laptop -> server -> hotspot -> internet

I have configured Netfilter on the server with this config:

#!/bin/sh

PATH=/sbin
EXTIF="wlan1"
INTIF="wlan0"

iptables -F
iptables -t nat -F
iptables -X

iptables -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE

iptables -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables -A FORWARD -i $EXTIF -o $INTIF -j ACCEPT
iptables -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT

iptables -A INPUT -i lo -j ACCEPT

echo 1 > /proc/sys/net/ipv4/ip_forward

qdisc are default one which are:

root@server:~# tc qdisc show
qdisc mq 0: dev wlan0 root 
qdisc pfifo_fast 0: dev wlan0 parent :1 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan0 parent :2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan0 parent :3 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan0 parent :4 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc mq 0: dev wlan1 root 
qdisc pfifo_fast 0: dev wlan1 parent :1 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan1 parent :2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan1 parent :3 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: dev wlan1 parent :4 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1

I connect to my linux router (server) with my laptop (debian 8)

Making a download test with wget on the server, I reach 1Mb/s, but when trying to test download from my laptop, I'm not able to reach that speed, It's seem to be limited to about one third of the total bandwidth of the server throught the internet, also the CPU load of the server keeps very low (<=1%)

tcp_window_scaling is enabled and net.core.wmem_max= 1048576 net.core.rmem_max= 1048576

I've digged for days now, and I'm not able to find a way to get full throttle from my laptop, Does anyone has some idea to solve this? I'm currently suspecting some bottleneck on the io bus...