When I first installed Rain Mobile the speeds were average. Then I put an external antenna on the roof and the speeds increased dramatically.
A few weeks back I suddenly started experiencing slow speeds again, and after a lot of effort in trying to get Rain to assist they finally gave me new tower co-ordinates to aim at. That worked okay, but as of a few days ago the speeds suddenly shot up again.
It appears to me that the towers get re-configured and either go slow, or fast. Sometimes you might need an antenna, or aim at another tower.