Would a range extender increase my Internet speed?

Would a range extender increase my Internet speed? I thought it would be, but in fact, it is not. My iPhone X only gets 40Mbps from the RE200 when connected to 2.4GHz, while it gets 70Mbps from my router’s 2.4GHz. I then contacted tech support and try to understand why the speed even decreased. The TP-Link support explained the RE200 performs well, and the speed is good because of the link speed and its working mechanism, duplex-mode, which I never knew before. Below I’ll post what they shared with me.

What is half-duplex mode? Firstly, we need to know half-duplex over-the-air modulation techniques. Half-duplex means that each party can communicate with the other, but not simultaneously; the communication is one direction at a time. Only when one communication is done, the next communication begins. All extenders adopt the half-duplex work mode.

What is link speed and how does it influence actual speed? Just like building a road before driving, network devices also need to establish a connection before they can communicate with each other. Link speed is like the capacity of the road; it determines the maximum number of cars on the road. Affected by road conditions, traffic conditions, traffic rules, etc., the actual number of vehicles will be less than the road capacity. The actual speed is lower than the link speed.

For the RE, the maximum link speed of 2.4GHz is 300Mbps. 300Mbps=150Mbps*2(stream)

For the iPhone X, the maximum link speed is 144Mbps.

In this case, if the link speed on the client is 144Mbps, in fact its maximum link speed is 72Mbps at one direction due to half-duplex. Considering the wireless interference factors (network conditions, client limitations, and environmental factors), the actual download speed is typically 40%-60% or a bit less than this in different scenarios. So the actual download speed on my iPhone, 40-45Mbps is acceptable. 

In short, the extender will not increase the speed but help to improve the Wi-Fi coverage.