Whatsapp messages not delivering

Hello,

Iam using Lenovo P2 device. It is at android version 7.0
I use whatsapp for my business purpose. So recently I created a group containing 140 members. Daily I post photos in that.
What I observed is when I post some 10 photos in that group, it is taking time to deliver those pics(clock symbol for so much time) and in the meanwhile if I try to send any message to some other person, it is not being delivered (always in grey clock symbol). But I will receive messages from others

Iam having a good wifi connection having 20mbps upload and 29mbps download speed. Other than whatsapp every other app(like facebook) works fine in this situation.

Each time Iam uninstalling and installing whatsapp to resolve this. But again it is taking me to the same situation when I post some pics in the group. If I send the same number of pics to an individual, it delivers very quickly.

First I thought it was my phone problem. So I started using Moto G4 plus and it is the same situation here as well. And today I heard the same problem from my business colleagues.

Kindly let me know how to resolve this issue.

Note: Iam using updated whatsapp version.

Thank you,
K Gopi Krishna