The thing is they just say "high speed service" in the contract, along with an "estimate" of how fast things will go.
Ergo: the contract does not guarantee you any specific throughput at all. Nor does it define "high speed", thus leaving that open for debate if you tried to leave. I'd bet they'd define "high speed" as 2mbit or so, anything else you manage to get is just lucky for you.
...so I guess you'd end up paying the termination fee after all
Note please that I can't guarantee this, it's just speculation having read the contract through!
Was on Xilo C&W LLU on MRCEN, Now Be* on MRARD.