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When i am using whatsapp application, my phone freezing and still on on whatsapp without any key working. What is the cause of this freezing?
Hi @Sako, welcome to our forum!
I'm sorry to hear about this. 
I'm afraid that the support for third party application is limited since we have not developed these applications and we can't guarantee full functionality, however there is a few things you can try.
When the application freezes, can you exit the application using the Task button (bottom right of your phone) and drag it to the left / right to close it or are you forced to restart the phone?
Does this happen every time you are using the Whatsapp application or can it be every fifth time? First of all, make sure that your application is updated to the latest version, you can check that in the Google Play store.
If the application is already updated to the latest version or there is no change after you have updated the app, try uninstalling and reinstalling it again. You can do so in Settings > Apps > Whatsapp > Uninstall, and reinstall it again from the Google Play store.
Let me know how it goes! 
I tried all of them and still have the same problem. I have Xperia Z5 compact. When I open whatsapp, It freezes for nearly 10 seconds then shows messages. It annoys me please help.
24GB used of 32GB.
Rather odd, could you uninstall/reinstall the app? make sure to download it from the playstore
I have done it again and it didnt work. It freezes sometimes.
Does it happen all the time, or when you have other apps open? or when you send/receive media?
It happens all the time when I have new messages on whatsapp and try to open them.