New Notification delay

May 20, 2013
Using wifi or 3G Whatsapp (every version) and some other social apps (facebook messenger etc...) give a notification delay of 10/15 min..... it doesn't happen every time....I really don't understand why...
I use Xiaomi mi2s with the last miui android rom.....but it happened also with older releses.
My wifi is always on, even in stand by.
I try to use PNF app but it doesn't change.