Most of the ISP in India i know upload speeds are capped 1:4, unlike other countries Like UK you get decent symmetric speeds uplink and downlink speeds, If your upload speeds low without VPN, then that is a limitation ISP, if the ISP uploads good then you have upload issue while connecting to VPN, then we need to config and MTU of the VPN.
So if this ISP side issue, you need to talk ISP or change the ISP. nothing much cisco community can do here.