According to this image, one needs 1h for boarding the bullet train. Is that accurate, and if so, why does it take so long?
There is an airport-style luggage, security, and ID check. Boarding itself doesn't exactly take one hour, just as boarding an airplane doesn't really require being there 2 hours in advance, but you need to plan for potential waiting queues, finding your way and walking to the platform, hence the advice to go there early.
As the other answer and comment explained, it is due to the size of the train station, as well as the security and ID checks.
However, on the particular example, I would like to just add that many highly lucrative (and competitve) routes, e.g. Beijing-Shanghai (example cited), have much smoother experience for business travellers (with a business class ticket or a VIP card/credit card benefits). If you know where to go (and follow security rules/have no special baggage), the boarding times in the graphic are much overestimated; you can probably do with 25 min for trains and 30 min for the airport. Even for economic class, the check-in process for these routes is highly automated as well for holders of Chinese machine-readable ID. For the highly automated and optimized Beijing-Tianjin intercity HSR route, I would even be comfortable with arriving 15 minutes before departure.
Beijing-South in particular has undergone some efficiency improvements (but the demand is increasing as well) with a lot of entry points and security check lanes now, which also means it might be more confusing to find your way if you entered in an inconvenient point.
Additionally, the trains and flights are run on an extremely high frequency (e.g. for Beijing-Shanghai, a flight every half an hour, a train every 10-20 minutes). Outside busy travel periods (e.g. Spring Festival), usually it is straightforward to change the train/flight to the next one (subject to ticket conditions, availability and fees) without losing much time if you miss it due to e.g. security checks.
Unfortunately, Chinese systems are usually not particularly friendly to foreigners travelling with a passport, although the automation also usually means the queue is shorter if you need manual assistance.
For people without a Chinese ID, first-time travellers who can't speak Chinese language, or even for Chinese people navigating a train station for the first time, as well during known peak travel times (e.g. Spring Festival, Golden Weeks, long weekends), the estimated time is probably appropriate.
In the end, the time needed varies depending on the routes , your familiarity with the stations, the documents you have, the time of travel and the overall traffic.
it doesn't take anywhere close to an hour. security check is perfunctory. first they check your ticket and your ID to make sure you're using your own ticket and that your ticket is valid. yes, you go through a metal detector and you get patted down, and your bag gets x-rayed if you have one. (it is not possible to check bags.) but they don't actually care if the metal detector goes off. from start to finish it doesn't take much longer than boarding a bus. maybe one minute more than that.