Utilizing onboard AND addon NIC card?

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CPU:  Integrated with Motherboard
Motherboard: ECS - NM70-I2(1.0) Mini ITX Celeron 1037U Motherboard
Memory: Corsair - Vengeance 8GB (2 x 4GB) DDR3-1600 Memory  ($73.89 @ OutletPC)
Storage: Silicon Power - Silm S55 120GB 2.5" Solid State Drive  ($45.98 @ OutletPC)
Case: Fractal Design - Node 304 (White) Mini ITX Tower Case
Power Supply: Corsair - CXM (2015) 450W 80+ Bronze Certified Semi-Modular ATX Power Supply  ($46.99 @ Newegg)
Wired Network Adapter: Intel - E1G44HTBLK PCI-Express x4 10/100/1000 Mbps Network Adapter  ($68.99 @ Amazon)
Total: $235.85
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Generated by PCPartPicker 2018-05-01 09:50 EDT-0400-0400

Onboard is a not so great REALTEK gigabit single port, and the addon card I installed is a 4-port NIC (which I later realized was overkill for my setup).

I'm getting okay-ish performance over LAN transfers, and currently my two interfaces are:

• LAN (port 1/4 on NIC)
• WAN (port 2/4 on NIC)

That's all I use, pretty simple.  The onboard realtek port is currently disabled, as I know most people say avoid it.  Just wondering though, am I losing out on free bandwidth by not utilizing the two separate physical interfaces? Or is any potential bottleneck coming from other factors (Could it be my unmanaged network switch, an 8-port $20 d-link switch?)?