Well in the UK, each channel broadcasts EPG data but this will take ages it's only now & next information. For the actual full EPG you only need to grab from a single channel ("Freesat Info" for Freesat, can't remember the name of the Sky one) so it's much quicker.
I'm actually using DVB-T Freeview. I could see it was going through the channels when it was doing the grab. I don't know if it grabbed from every single channel as I wasn't watching it but each time I looked at it it was on another channel.