Hello  @J.Francisco 

Thank you for creating a detailed post! The high latency may be not caused by the range extender,   to help isolate the issue, you could test as below: 

1. check if high ping/ latency issue is only with certain device: test other network device like a different PC/ Laptop/ mobile phone 

2. Check if router's Wi-Fi network has any latency or not: at range extender's location, connect to router's 2.4Ghz Wi-Fi band to test if there is any latency

3. Wire a computer to Range extender by Ethernet cable and see if there is any issue

4. Does this happen when you run any certain appliances? It is recommend to  close unnecessary apps when test ping/ latency. 

Here is guide to check the latency: What should I do if I get the high ping value from TP-Link Wi-Fi router?

How did you check the latency, can you share the photos of that? Does this happen with wired/ Wi-Fi connection on RE. 

With RE unplugged, test the router's 2.4Ghz Wi-Fi latency as well, and share a photo. Thank you. 

Hello  @J.Francisco 

Can you share some test result as requested above? 

When connect to TL-WA855 WiFi network, you could try to ping RE's IP address, ping router's IP address and ping 8.8.8.8. 

If phone connected to RE's Wi-Fi, if you could get a low ping when ping router's IP address,  the communication process between RE and router is fine.

How to Use the Ping Command

Hello  @J.Francisco 

• May I know can you run an Ethernet cable between router and RE and use RE in Access Point Mode to test connection? 

That could help isolate the issue and check if it is related to bad Wi-Fi environment (walls ,obstacles, wireless interferences etc)when the signal being transmitted  from router to RE. Wireless connection, especially 2.4Ghz Wi-Fi could is usually crowed or in congestion.

• Try to locate RE closer to router 

How about the ping test of router's WiFi at RE's location? Try to ping router's IP and 8.8.8.8 when device connect to Wi-Fi network. 

Please also check if a mobile device could surf internet as usual or has lag issue 

Can I know the model of WiFi router, and network topology. e.g.  router on first floor <XX foot> RE in bedroom <X foot>Wall <Wireless >PC & phone has latency