Cannot get more than 300Mbps from LAN to WAN. is that it's limit ?
Yes, limited by CPU processing speed.
LAN <-> LAN All low level layer 2 network frame processing is done by the switch chip, no NAT no routing, no filter/firewall, less CPU power involved.
WAN <-> LAN by software NAT, firewall & filter, Routing (gateway mode) everything adding CPU load.
Router "Main" chips may integrate some chipset specific acceleration that is not documented, not open source and therefor not usable by dd-wrt, openwrt etc only by the licensed "driver" software from the chip manufacturer that is inside the device factory firmware.
The same user unfriendly stuff you see with graphic card drivers on unsupported OS, no or less acceleration compared to manufacturer drivers...
