These performance anomalies are common in the US with the major carriers. When a location is accessed by too many native users, the service speeds suffer greatly despite the signal strength. I believe the roaming services typically default to the strongest signal despite the available bandwidth. If that carrier is overloaded by local users, your roaming lower priority will suffer greatly. Switching to another available roaming partner frequently solves the problem.

Here at home, Verizon actually offers a priority upgrade for a fee to their users, giving them first access to the available bandwidth.