You need to understand that the so-called HSPA+ is radio network, i.e. from your phone to the radio base station (RBS), which you can see on top of some buildings.
From RBS to core switch is the trunk.
The core switch is connected to the Internet.

The fact that the radio network is fast doesn't mean the trunk is also fast; and the core switch connection to the Internet is another matter.
So, where is the bottleneck? It could be at the radio network, trunk, or beyond the core switch.

AFAIK, XOX uses Celcom's radio networks and trunks. But it MAY have its own connections to the Internet (I am not so sure).