Are you pertaining to slow internet mobile connection you usually experience when afternoon comes, unable to make a phone call, or send text message?
I'm not quite sure about this one, but I think it has something to do with the volume of people using those services (mobile internet, phone calls, text messaging).
Most office workers usually go out of work in the afternoon. Most kids go out of school in the afternoon. So what do you usually do after school and work, specially we all know most of those two places restrict the use of mobile phones when inside? You use your mobile phone, right?
Have you also noticed that afternoons are included in rush hours? Say when you're stucked in traffic, or waiting in long line at an MRT Station. Notice what most people do. They fiddle with their phone.
Send a text to Mom that you'd be late from school because of traffic, or post a photo of how congested Edsa is on social networking sites, or call your someone that you won't make it on time.
And also, I guess it has something to do with the weather. Nowadays we experience a hot weather almost the whole day, and at times rain follows in the afternoon. And I think weather, and abrupt weather changes to be exact also affects telecommunications, such as performances of transmission towers.
Above statements are only my theory though.
These facts are true, also with non globe subscribers