We need wait 3 minutes to have access to internet after authentification on Wifi

Hello,

My users when is connect the first time on network Wifi Sophos they need wait 2 or 3 minutes for having access to the internet.
Is a real issue for the guest people is coming they think it doesn't work.
I have the same issue on two different site.

I have many AP100c managed by a firewall sophos xg125.

Do you have any idea why we need waiting a long time ?

Thanks in advance for your help