@rottenpeanut
Hi, for your issue, please take the below answer:
Due to the wireless transmission theory, the range extender is set to half duplex communication instead of full duplex, in other words, the range extender has to process one way communication between your root router/AP and the terminal clients, so the transmission time will be double increased, on the contrary the
wireless link speed will be half decreased. The real download speed is about half of the wireless link speed, so maximum download speed you can get is about ¼ of the wireless link speed as to range extender.
For example, if your main router/AP is 300Mbps wireless router, and you are using 300Mbps wireless range extender WA850RE (the 300Mbps mentioned is the wireless link speed), theoretically you can get 75Mbps download speed. But there are wireless interference, the real download speed cannot get only approach 75Mbps.
If you think the speed from the RE is slow on your device, you can follow the instruction to check the wireless link speed between the RE and your clients first.
https://www.tp-link.com/support/faq/2265/
Hope it can help you.
Good day!