I'm pretty sure that this has been going on for a while and became much worse once free roaming started being offered. I've tried using Three and Vodafone abroad with connection speeds often as low as a few 100kB's.
The first few of web pages usually work fine, beyond which it's like swimming through treacle. Switch to another device and all is fine, until you hit the throttling again. It's not consistently bad, but when it's bad, it's very, very bad.