Consumers of DSL services have the most reason to complain according to data in the FCC’s fourth Measuring Broadband America report out today. The study says that during the evening when usage is highest, 95% of Verizon DSL customers received an average of 42% of the company’s advertised download speed — the worst performance among the Internet Service Providers (ISPs) studied. The FCC plans to send letters to the CEOs of Verizon as well as Centurylink, Frontier DSL, and Windstream to find out how they plan to improve their performance. “Consumers deserve to get what they pay for,” Chairman Tom Wheeler says. “While it’s encouraging to see that in the past these reports have encouraged providers to improve their services, I’m concerned that some providers are failing to deliver consistent speeds to consumers that are commensurate to their advertised speeds.”

Ten of the 14 ISPs studied slightly improved their performance since last year’s report while others showed little change. Still, it notes, about a third only offered 60% or more of their advertised speeds 80% of the time to 80% of their customers. “This is a metric that we expect ISPs to improve upon over the course of the next year,” the study says.

Among the largest ISPs, 95% of Comcast customers received 98% of its advertised download speeds , with Time Warner Cable, Charter and Cox at at 93%. AT&T trailed with 62%. But some ISPs exceeded their advertised speeds: Verizon Fiber delivered 101% and Cablevision had 117%.

Later today the FCC will release data about congestion that it found in different points in the network. That could be relevant to the debate that Netflix helped to ignite about whether some ISPs including Comcast and Verizon are unfairly requiring Web sites to pay to guarantee smooth transmissions. Wheeler recently said that the FCC will study this subject.