TCP Throughput Limitation in a simulated WAN Network

I have the following WAN network topology in which I simulate it using Network Simulator ns3:

Wireless Station <---Wireless Link---> Access Point <...Wired Link...> Router <...Wired Link...> OnOffApplication

1. The wireless link uses 802.11ad and supports up to 5 Gbps with Two Level Aggregation (MSDU + MPDU Aggregations).
2. The wired Links are Full Duplex Ethernet with 10 Gbps of Bandwidth and 20 ms of propagation delay.
3. The OnOffApplication uses TCP as transport layer. It generates data with a rate of 4Gbps and the packet is 1448 Bytes.

The OnOffApplication establishes a TCP connection towards the Wireless Station using Cubic Variant and I keep the application on On State to keep always sending packets. However, despite the high bandwidth I have, the perceived throughput by the client is very low and it is around 25 Mbps. I have tuned the receive buffer on the two ends to support up to 4 MBytes. So I assume the transmitted will be limited by its congestion window.

So what's the limiting factor of the throughput in my simulation? Is there any parameters that I need to tune?

Note: When I decrease the delay on the wired link, I start to get high throughput and I can reach up to 3 Gbps for a delay of 0.1 ms.

So why TCP is so affected by propagation delay?