The Mamiya 645 Pro I had was very fast to reload if you had extra inserts loaded ahead of time. It also allowed quickly switching backs. The Hasselblad is fast to switch backs, but doesn't use interchangeable inserts. Both are a bit slow to load the film into the insert however.

The Mamiya 7 is kind of like loading a big 35mm SLR. It's not fast, but not too slow either (probably about a minute to load new film, but I haven't timed it). You get the hang of the all of these cameras and with practice you can get pretty fast.

The cameras with interchangeable backs are by far the fastest way to keep shooting, at least over span of a few rolls (assuming you buy a few backs or inserts and load ahead of time).