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If you set your LAN interface to run at 100Mbps, likely you would just be moving your possible bottleneck one hop deeper into your LAN (often on devices with weak QoS support).
Leave your LAN interface at gig. If congestion, on WAN egress, becomes an issue, you can then try QoS to mitigate it. For example, for starters, you might want to place your WAN egress traffic into fair-queue.
