Do you know what the wireless standard of the devices receiving/sending are? (Wi-Fi 0-6). The speed of WiFi could be limited by the standard of WiFi hub and receiver you've got and getting an extender might not do anything for you.
Check the model of your WiFi hub for Wi-Fi standard. With the other device, if its a laptop you can go into devices to see the WiFi standard and find the speed limit of it.