Internet browsing slows down on GL X3000 vs Phone

I had a similar issue, but only when using VPN. I had to change the mtu to 1280 and speeds are back to normal.

When you calculated the MTU, the modem probably was not set to 1500. Therefore the value you got through pinging was not accurate.

Although you see the mtu unset (blank) from the webpage, when you ssh into the box and run ifconfig you will see the actual value set on the wan interface(rmnet_mhi0); I have flashed few different firmware versions and found out that the value was variably set by GL engineers! 1428, 1472, and 1500.

I would advise you to set the MTU again to 1500 then repeat the pinging process to find the most accurate value.