proxy squid behind mikrotik VPN (ppptp bridge mode) slow

hi … i need advice

i have completed deploy VPN site to with mikrotik (using ppptp with bridge mode) .
both site can ping each other , i can access file sharing and internal website.

But i have problem using proxy server (Squid Cache: Version 2.7.STABLE9) in remote site .
I see the access.log of proxy the pc in remote access but in remote site pc the web site page can’t display and take long time event no show up and time out but no problem in internal website.

the remote pc using adsl connection with 512/128 (upload) and get ip address from internal dhcp in head office.

any ide to tune up my vpn because i want remote site using our proxy in head office for all internet connection .

many thanks