When the problems occur — and they still do — the command center sees them first, in charts that suddenly spike on the television monitors. The data also serves as a reality check in a hypersensitive media environment. Last month, CNN reported that HealthCare.gov had gone down again. A quick look at the screens made it clear that whatever the problem had been, it was fleeting.

Mr. Zients’s metrics, meanwhile, are improving. When the repair effort began, response time — how long it takes a page to load — averaged eight seconds; now it is less than one. The error rate — how often users are unable to click through to the next page — was 6 percent; now it is 0.75 percent. When Mr. Dickerson announced that the day had ended with no major crashes and no one who could not log in, the engineers erupted in applause.

“That’s the job,” he said. “When things break, you have to fix them.”

But even as the White House points to its progress, the administration on Wednesday said troubles with HealthCare.gov had forced it to delay, by one year, an online exchange for small business.

Other people working on the project, speaking anonymously because they are not authorized to talk to reporters, say significant challenges remain.

Some of the companies building the system opposed an early decision by the Medicare agency to use database software from a company called MarkLogic, which handles data differently from systems by companies like IBM and Oracle. Some suggest that its unfamiliar nature slowed their work. By mid-November, more than six weeks after the rollout, the MarkLogic database — essentially the website’s virtual filing cabinet and index — continued to perform below expectations, according to one person who works in the command center.

In interviews, MarkLogic’s executives faulted inadequate computing power and instability at the site’s data center, as well as the failure to properly integrate their product, problems repeatedly cited by other website vendors.

But perhaps most important, it remains unclear whether the enrollment data being transmitted to insurers is completely accurate. In a worst-case scenario, insurance executives fear that some people may not actually get enrolled in the plans they think they have chosen, or that some people may receive wrong information about the subsidies for which they are eligible.