Thanks for your reply.
One thing you can try is to login to router's WebUI, navigate to Network -> Firewall -> General settings and enable Software flow offloading option. - As advised this is done. How will this help?
Could you provide more details about what type of VPN are you using, what encryption algorithms, other settings do you have configured? - Using the RMS VPN.
Are you using mobile or wired WAN connectivity? - Using wired WAN connectivity
What speeds do you normally get, without VPN? - Did a Speed Test and "DOWNLOAD" - 208.40 Mbps and "UPLOAD" - 305.35 Mbps