Check if offloading is really enabled.
"show ubnt offload".
When there is a process or setting that prevents offloading, it can be disabled even when it is enabled in the config.
You should see something like this:
XXX@ubnt:~$ show ubnt offload IP offload module : loaded IPv4 forwarding: enabled vlan : enabled pppoe : enabled gre : disabled IPv6 forwarding: disabled vlan : disabled pppoe : disabled IPSec offload module: loaded Traffic Analysis : export : enabled dpi : enabled