@MingShow 

It does sound like latency on the wireless backhaul between the nodes, there are 2 things to try first

1.  Run network optimisation to see if it will move you to another channel.   Link below page 18

2.  Turn on QOS and set your PC as a priority device, this will consider your gaming traffic over all else.   Again link below page 26

https://static.tp-link.com/2021/202104/20210429/1910012954_Deco%20M5_UG_V1.pdf

I would give those a rattle and see how you get one.    This simply could be that a neighbour or someone close has a new router or AP and its causing that bit extra interference with you.  Seen it before where it worked fine for years then someone changed something and now you are affected as a result, last time I had this with a customer on Eero (similar to deco) we just bought an additional node to act as a middle man between the main node and him, that extra jump point increased the speed for him back to what he had. 

It literally was just the additional interference caused his signal to drop a fraction and it became noticeable.   May be the same for you, you could try something like WiFi analyser if you have an Android phone, it will tell you what is going on around you.  

https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer&hl=en_GB&gl=US

@Philbert 

Thank you for those recommendations. I had my PC set up as one of the priority devices on the QOS, but I have now removed the other 3 devices that were on there so my PC is the only one. I'll see if that can improve things.

I had also run the network optimizer a couple of times recently but haven't seen any improvements there, but I'm giving that another shot.

I also opened a ticket with my ISP to see if there's any issue on their end that might be causing problems, so hopefully I can get to the bottom of this.