When you say "20 seconds before the message shows up on my screen", what precisely do you mean? That you get the notification but it's 20 seconds for a pop-up to appear, or that it takes 20 seconds for the message app to open, or...?
Hard to say more without knowing precisely the symptom, but my first thought is to ask what messaging app you are using, and whether you are using anything else that might affect this (lockscreen apps, SMS popup apps, other messaging apps, security apps which respond to SMS commands)? And is it any particular type of message (e.g. stuff sent as MMS rather than SMS)?
If we can't work it out it might be worth backing up your data and doing a factory reset, because over the air updates can sometimes cause odd things. If rooted, try erasing cache and dalvik and see whether it improves.