The official 1940 census website was completed overwhelmed and experiencing many problems yesterday but appears to be working today. I noticed images of census pages load completely different than they did yesterday. Yesterday an image would load as a bunch of tiles to make up the entire image. Today an image is loaded as just a single file in about 10 seconds for me. Hopefully it can handle the load this evening.
There are other websites that are also adding census images. Family Search has added Alabama, Colorado, Florida, Kansas, Oklahoma, Oregon and Virginia. Here is a Family Search shortcut link to their 1940 census. Ancestry has American Samoa, California, Delaware, DC, Guam, Indiana, Maine Nevada, New York, North Carolina, Ohio, Canal Zone, Pennsylvania, Tennessee, Texas, Virgin Islands, Virginia and Washington. Here is a Ancestry shortcut link to their 1940 census.
A third website has started loading census images; MyHeritage. It appears that they have images for about 44 states already and should be complete by tomorrow. They are also the the first website to have indexed records but they are only or Bristol County in Rhode Island. Here is a MyHeritage shortcut link to their 1940 census.
You should test each website’s method of displaying a census page and see which you like the best.