WordPress.com, used by over 13 percent of the 1,000,000 biggest websites, was hit today by an "extremely large" distributed denial-of-service (DDoS) attack. As a result, some users experienced performance and connectivity problems, Sophos.

The @wordpressdotcom Twitter account has sent out the following messages recently:

Some people have reported slowness on WordPress.com. We're working on this and restoring normal service to everyone as soon as possible.
about 10 hours ago via CoTweet
Service should be back to normal now - back to blogging, everyone!
about 9 hours ago via CoTweet
Sporadic slowness is back – we're battling a non-trivial DDoS attack and will update as soon as it's mitigated.
about 3 hours ago via CoTweet
Our systems are back to normal. We'll continue to monitor them and post updates here if needed.
34 minutes ago via CoTweet

Due to the size of the attack, which was multiple Gigabits per second and tens of millions of packets per second, Wordpress.com struggled to mitigate it immediately. In fact, the attack was large enough to impact all three of the company's datacenters in Chicago, San Antonio, and Dallas.