Slow Wifi Internet access for Macbook Air

Hi

I have a client who roams from one site to another
He owns several Apple devices: iPhone, iPad, and MacBook Air

All their sites are equipped with RB951G-2HnD

His Wifi Internet access using the Macbook Pro is always slow
e.g. six seconds to load Google’s search page

In comparison local Windows laptops have fast Wifi Internet access

This is the common Wireless setup for all sites:

/interface wireless
set [ find default-name=wlan1 ] band=2ghz-b/g/n channel-width=\
    20/40mhz-ht-above country=france default-authentication=no disabled=no \
    distance=indoors frequency=2462 l2mtu=2290 mode=ap-bridge \
    security-profile=XXX ssid=NXXXXXX01 wireless-protocol=802.11

/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
add authentication-types=wpa-psk,wpa2-psk eap-methods="" group-ciphers=\
    tkip,aes-ccm management-protection=allowed mode=dynamic-keys name=XXX \
    supplicant-identity="" unicast-ciphers=tkip,aes-ccm wpa-pre-shared-key=\
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX	\
    wpa2-pre-shared-key=\
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

The SSID is solely composed of capital letters, plus two digits

I recently set the Wireless-Protocol to 802.11
Before it was set to any or unspecified
But the user hasn’t noticed any improvement

Any ideas ?
thanks
yann