Performance will rarely be deal breaking. Instead people will toil away with slow systems and just accept it. It doesn't mean that they wouldn't be much happier with a faster system.

We recently replaced a system whose online store took about 900ms to respond, and their back end admin section on average around 1.7 seconds. It obviously wasn't deal breaking since they lived with it for 3 years but it still was still ridiculous, and definitely had an impact on both their productivity and bottom line.