The official 1940census.archives.gov website is still experiencing speed issues. NARA has said that the issue is one of server capacity, and their partner Archives.com is working to add more servers to handle the traffic. USA Today has an article about it. In the meantime, colleagues have informed me that if you do a search on the official website for an enumeration district, then try downloading the images, which is working much faster than trying to view the images online.
Ancestry.com and FamilySearch.org are working to scan the microfilm and get them processed as quickly as they can. as of 3:30 p.m. EDT, FamilySearch.org has the states of Delaware, Kansas, and Virginia online. Ancestry.com has the following locations online: American Samoa, Delaware, District of Columbia, Guam, Indiana, Maine, Nevada, Panama Canal, Rhode Island, Virgin Islands, and Virginia. the states of California, New York, and Pennsylvania are currently in process.