"All T1s are not created equal"
Well, actually, the T1 local loop is going to give you 1.5 mbps, yup, it is. Then it's going to some central location where they're going to stuff too many T1s into a larger pipe. This is also part of "the great Lie of DSL:" you get dedicated bandwidth, but once it hits the Central Office (the "switch room"), it all gets stuffed into the same common pipe, usually with a great deal of oversubscription. COVAD is notorious for massive oversubscription ... in Chicago, SpeakEasy split from COVAD and built their own POP so they could get their service levels up.
Some oversubscription at the points of aggregation is normal business, HOW MUCH the big pipe is oversubscribed is what makes a good provider versus a bad provider. If you toss in some additional oversubscription for Frame Relay, it's going to slow down periodically.
Check out the Service Level Agreements (SLAs). If you want better service (less latency, less congestion, less oversubscription) it'll cost you more.
UUNet is a premium provider: They own more of the Internet plumbing than anyone. They're top-tier (they connect directly to the "big pipes"). Smaller, cheaper ISPs / providers are usually NOT top-tier. they buy bandwidth from someone that buys it from a top-tier company. At each level, the owner of that chunk of bandwidth divides up the bandwidth with (usually) some oversubscription.
I could set up a DS3 MUX in my house, and sell all of the neighbors a T1. Then I'll route all those T1s through my MUX and into my cable modem (with a whopping 256K uplink). They all would have T1s, but they'll NEVER see anything above 256K of throughput (end-to-end).
The key is to figure out what service level you need to get the task done, then find a provider that'll guarantee that service level for the best price (and other features). If that's "too expensive," your two choices are to reduce your needs, or pay up.
Service level includes availability (how much down time), performance (throughput, latency, etc), and other possible features like security, 7X24 Network Operation Centers, monitoring, maintenance ...
Like nearly everything else, "you get what you pay for."
JM .02
Scott