Or join the arms race and get a bigger more powerful one to swamp them out !
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The Realtek RTL8821CE is a single stream 1 x 1 Wireless AC adapter , so running on a 40Mhz wide channel which your graph is showing, will give speeds of around 75- 85Mbps.
To run at around 200Mbps (close to the Hub) on this device you need to run a 80MHz wide channel in AC mode. I would double check that the Hub is set to ac\n mode with the 80/40/20 channel width enabled, bearing in mind that you will need to use a channel away from other networks so you can use the 80MHz wide channel unhindered.
The iMac 2011 I believe is a wireless N device so, max channel width would be 40MHz, (worth checking if the wide Chanel is enabled on the 5GHz network) but i believe it is MIMO with either 2 or 3 5GHz antennae so should be capable of around 150 - 170Mbps n the 5GHz band. (more if it is a 3 x 3 MIMO)
The iPhoneX is a MIMO wireless AC device so should be capable of around 150 - 170Mbps even if the channel width drops to 40MHz, much faster with the 80MHz wide channel.