Yes, you are correct it saying that if it only worked 50% of the time it would be unacceptable, but, running at a much lower speed for a short period of time is not the same.
I have a retail background so I'll use a retail analogy. If you buy a cell phone, and it is not working, and return it to the place you bought it from, the best way the shop can sort out the phone is send it in for repairs. The customer will not be 100% happy about this because they bought a product and rightfully expect it to work, but the best the shop can do it try and fix the issue and not go back in time and prevent the fault.
The fact is that we, as customers, should expect everything to work 100% of the time, and as long as the relative service provider is willing to try and improve the service or fix the issues we are experiencing that is good enough for me. It's when I get the answer, "I'm really sorry sir, there is nothing that I can do to rectify the problem, your phone is now broken buy a new one" that I find it unacceptable.