Something I'd encourage people to do when roaming internationally is to set your phone to restrict background data transfers.

On Android, Settings > Data Usage > (three lines) > Restrict Background Data.
On iPhone, ??? I'm sure it has a setting for this.

It's amazing how much of a difference this makes on performance when you're running at 2G speeds. I was cursing at 2G speeds when I was last in London until I realized how much data was being used when I wasn't actively retrieving anything.