VPN Clients in the internal network have slow VPN traffic to external Servers

Sophos SG 9.502-4

500MBit/50/Mbit

We have clients in the internal network which use Open VPN (UDP 1194) client connections to external servers.

But the VPN traffic is slow over the sophos sg firewall (3Mbit). With the same client computer we get normal results outside the network.

The Sophos SG slows down the speed, even if we switch Intrusion Prevention off.

Thanks for any help