For speed rate I'm not sure but maybe you could make a script that would recognize background process and then activate the 3G-GSM change. Then you can use it as custom action in profilmatic... and use "mobile net connection" and time interval as condition for that action (I prefer to use "Idle status" instead of "time interval" because periodical scan is wasting of energy and I don't need change of status when I'm in idle... so I usualy put script to be execute on Idle status change).

Only condition for this to work is to have "Allow background connections" On and "PSM" Off... so profilmatic can detect you use GSM network to connect on Net and not wifi.