Why did Wi-Fi speed fall after replacing the router with TP-Link Archer C6?

Good afternoon. Problem. There was an old router DIR-300. In neighboring rooms on Wi-Fi poorly received the Internet on a computer, tablet. I decided to buy a modern router with good characteristics. Took TP-Link Archer C6. Connected, configured, the Internet appeared. The cable speed is excellent about 100 Mb/s, as the provider gives, but on Wi-Fi the speed became terrible: then 10-12 Mb, then in general 1-2 Mb. With the old router speed was my more, but I did not measure on the old one. Can you tell me if I need to change something else in the settings? Although it seems that everything was set up according to the instructions.

Answer

Good day. I have a few questions that need to be clarified:

How do you check the speed? On what devices (you need to check on different wi-fi devices)? Did you measure close to the router?
What Wi-Fi network settings did you change when configuring TP-Link Archer C6?
Did you measure the speed via cable from the router or directly?
What Wi-Fi network (in what range) are the devices on which you measured the speed connected?
Are there a lot of neighboring Wi-Fi networks?

100 Mbps in the 2.4 GHz range is almost unrealistic. Rooter will cut the speed anyway. Even in the 5 GHz band. But 10 Mbps is a very low speed.

Try to apply the solutions from the article: how to increase the speed of the Internet on Wi-Fi through the router.

It may be that you have strong interference there, which is why the speed dropped so much after replacing the router. Perhaps the speed was the same before, through the old router, just you did not measure.