Hello and thank you for contacting us, this is Martín, Tech Support Engineer for Teltonika Networks.
Technically, the RUTX11 does have Gigabit interfaces, however, the traffic processing inside of the device can partially bottleneck the transmission. The WAN traffic requires Routing, Firewall and NAT, and there is an expected CPU performance impact on the device when using the WAN connection at such high speeds. Software Flow Offloading can help as this disables QoS and SQM features, but still it can't enable the full speed usage from this connection.
Answering the title of your question, the speed we could test from LAN to WAN in the device trough iperf3 was an average of 888 Mbit/sec when enabling port LAN3 to be used as a WAN with a factory reset device, so, the speed you reported is the range of the expected speed trough this mode.
I remain attentive to any further comments.