Device needs to be snmp RO at least and drill down to the interface traffic charts bps in/out. Given the interface is manually configured with a 20M limitation your chart should scale. (if not it will be full interface speed and you can still get the bandwidth, just not in proportion to your paid for 20M)
For the NPM being slow I have seen the biggest effect on response directly releated to the DB size.
in System Manager - go to the Node then the interface, right click and select Interface Details.
here's an example of a Metro Ethernet interface. you could see about the same from your Orion web site. just drill into the Node then the interface.
you should see the Util and the rate of your 20MB link like this one shows. we're doing 9Mpbs on the 20Mbps in this snapshot.