@Johnny88lai
Hello, is the computer connected to the 5G network when you are playing games? Are the 2.4G and 5G sharing the same SSID? If the Smart Connect is enabled and 2.4G and 5G are sharing the same SSID, try to disable Smart Connect and set up different network names for the 2.4G and 5G, then connect the computer to the 5G network to check if the speed stays stable.
You can also try to modify the wireless settings on the router to find a less congested channel if the speed drop is caused by the interference of the networks. Go through the following article to review the settings on the router:
https://www.tp-link.com/blog/95/10-ways-to-boost-your-home-wifi-for-a-faster-internet/