Since upgrading both my 3GS's to iOS4 I get this delay ( carrier update was also applied), even with full signal.
I also find that my email will show only the header when I click on it and no message in the body. I have to go into the background apps shut the mail off restart it and then the full messages will show. I have to say this has been the worst software release since the lagging keyboard in OS3.0. I also have dropped calls which in almost threes years I haven't dropped any (I'm on Rogers in Canada and the network is pretty good). I'm sure we'll get a fix for all this stuff, just hope it's sooner rather than later.