Ooma slows down data traffic

It's been a month since I bought the Ooma VOIP system. It's been working fine, except with some issues, like the scout hang (need reset it). My configuration is to put Ooma hub right after DSL modem, so my wireless router is connected to Ooma hub. This is per suggestion in its manual.

I was curious to see how data traffic was affected. My nominal DSL speed is 6 Mbps, and when the router was connected directly to dsl modem, I could get more than 5 Mbps average. But when the router is connected behind Ooma, I could only get below 5 Mbps. It is not strange, as Ooma was acting as a NAT router too, hence added additional overhead.

The settings showed on setup.ooma.com (an alias to its internal IP address) look very similar to regular NAT router. The default internal IP address range it uses is in 172.27.35.*

Ports used:

Telephony:	50 - Running
DNS:	45 - Running
Web Server:	47 - Running
VPN:	356 - Running
Free:	37008

Next experiment I will do is to put packet sniffer on its "modem" port to figure out how it actually works.