High Latency Issues in Local Network

Hi Everyone, i had some issues with my localnet latency.

I have a network layout scheme for 13 rooms, consisting of:

• 1 master router: Ruijie RG-EW3000GX
  
• 12 slave routers: Ruijie RG-EW1200G PRO
  
  

All devices are connected via a mesh network and operate locally without an ISP internet connection (im just using the router for local network and pbx asterisk. My client didn't want to use ethernet cable for connection between router) .
the problem is I'm experiencing latency issues and some how i got high ping if im doing pbx call (via asterisk)  and opening some local webapp.  
how can I solve this thing?

Dear sir
Can you provide the following information?
1.What is the wireless speed measurement result at the end point?
2.Is there any delay when wired devices are connected to the network? If so, what is the delay?
3.How many users are connected during the test?
4.Are you testing 2.4G or 5G?
5.If the device is online on Ruijie Cloud, can you share the network to this email v-yuchen3@ruijie.com.cn
?