VDSL WAN Throughput issues R510

[jon_kalter]

VDSL/ ADSL WAN Throughput issues on R510. Does not achieve Ethernet verified speeds of 140 megs down from Internet, struggles to hit 50 in both Gateway and AP mode. 

[matt_236689]

Have you run an MTU test?  When people mention DSL, that usually seems to be the issue.

[jon_kalter]

Thanks Matt, I've run a ton of packet captures and nothing is above 1492 (Qworst modem is set to 1492 MTU but where is this setting on the R510?) I've poured through the 104 and 106 user guides and MTU is not mentioned anywhere...... Is this set via cli? If so I assume SSH or Telnet works? 
Such a strange issue.....