Official test redults indicate that this device can route at 300Mbps (give or take) depending on configuration … most often the number under “routing - 25 filter rules - 512 byte packets” resembles reality the best. While it is possible to squeeze more throughput out of this humble device I highly doubt it’s capable of getting anywhere near 1Gbps.
Your particular use case (PPPoE) does reduce throughput considerably on many device models, hence current throughput you’re seeing (200Mbps) seems to be quite right. Much more powerful device is needed to get near your desired goal … look at RB5009, RB4011 or hAP ax3 (if you don’t mind getting wireless device).