No surprise at all what your experiencing.
Boost uses the Sprint network and the entire network nationwide is completely overwhelmed. Posts from Boost, Virgin and Sprint postpaid customers (here and in various forums across the Internet) are all the same everywhere in the US - horrible to less than dialup data speeds.
Sprint also has the smallest network of the Big 3, and relies on ALOT of Roaming with Verizon to fill in coverage.
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funny you mention roaming with/onto VZW, I was upstate NY with Boost and in a complete dead zone for Boost but my friends VZW worked fine, so I guess "peering" agreements are dependent on the area.