From the MIT Technology Review:

In particular, the project was doomed by a relatively late decision that required applicants to open an account and let the site verify their identity, residence, and income before they could browse for insurance. That meant the site would have to interface in real-time with databases maintained by the Internal Revenue Service and other agencies.

“You could put 100 Google engineers on it, and it’s not going to fix [the fact] that the scope of the project is flawed or fix the IRS system if it’s slow,” says John Halamka, chief information officer of Beth Israel Deaconess Medical Center in Boston. “You don’t want to query 10 downstream systems and be reliant on their performance, because you are only going to be as good as the slowest one.”

This problem did not plague many of the health-care exchanges at the state level. For example, the Massachusetts system lets you shop for insurance anonymously, and even apply for an insurance policy without setting up an account. Identity confirmation is done later, before the policy becomes effective. Similarly, the exchange in Kentucky—which, like the federal website, went live on October 1—didn’t require users to set up an account or verify their identity before enrolling.