@Ash1 

Hello, Hope you could check this article already:

Low speed with Range Extender

Additionally, please help check some details and we will try to help you out:

1. How far away between the main router and extender? What about the signal strength you can get on your phone if standing next to the extender?

2. With a computer connected to the extender, please check the wired/wireless link speed: https://www.tp-link.com/support/faq/2265/

If you can get a good or adequate signal/speed from the main router in the location of the extender, there might be no need to add an extender. What the extender does is extending the wireless signal to a dead zone but not speeding up.

Hope it makes sense. Any updates, please let me know.

@Ash1 

Hello, the speed would be slower or even half when connected to the extender network as the extender is actually working in half-duplex mode, which means the extender will need to receive the signal from your main router first and then transfer the signal to your computer or phone and this cannot be happening simultaneously.

Hope you may understand and accept.