push notification iPhone 3G

When using twitter app, facebook app, Whatsapp  and beejive im,  i've noticed inconsistent and slow push notifications on my iphone 4 when the apps are not active or the phone is on sleep mode, almost 5 or 10 minutes or they never arrive, and  I don't know is this issue is common, because in my Blackberry the msn, twitter and facebook notifications arrive almost instantly, ¿is there a way to speed up those notifications? ¿is that a problem of the iOs design, or the app design? I'm using an iphone 4 with iOs 4.3.3 (8J2), and this problem is really annoying