Networking :: VPN Server And Internet Gateway

I just set up a linux server which is acting as an internet gateway. For specific reasons, clients first make a vpn connection trough the internet to the linux server.

CLIENT ---->Internet----->Linux VPN Server ------> Internet

On the linux server iptables is configured for NAT. The problem is that the internet speed becomes slower than expected. I used windows and RAAS and the speed was pretty good. The server has only 1 NIC(eth0) and an alias interface is added to eth0. The alias is eth:0 with ip address 192.168.0.253

iptables config is as follows:
iptables -P INPUT ACCEPT
iptables -F INPUT
iptables -P OUTPUT ACCEPT
iptables -F OUTPUT
iptables -P FORWARD DROP
iptables -F FORWARD
iptables -t nat -F
iptables -A FORWARD -d 192.168.0.0/24 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -s 192.168.0.0/24 -j ACCEPT
iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -j SNAT --to MY_VALID_IP