I inspected your router and configuration and here is what I found:
1- Here are the performance statistics of your router: RB4011iGS Test Results
2- You have a pppoe-bridge set up on your router. This can cause CPU bottleneck.
My advice is to remove the pppoe bridge and create a pppoe server for every interface that you need:
Edit your router and go to the Config tab (screenshot for reference):
Click Add PPPoE Server and select the desired interface.
Repeat until you have added all the interfaces that your clients will connect to.
3- You might want to enable Burst on your service plans:
Feel free to try these suggestions and let us know your results!