I see these way too often, and most operations really should feel instant to the user. Little things should not be taking multiple seconds to complete.
And for the occasional actually-time-consuming task, calculate the end point so you can display a proper bar. An indeterminate spinner is laziness, forcing the user to wait an unknown amount of time when you can probably figure it out.
Local (client-side) operations are a different matter entirely. Either precompute the result for common operations or optimize for speed!
As a general rule, every single edit should be able to return control to the user immediately. If the network can’t be reached, save locally and attempt an update in the background (showing a small, unobtrusive annotation that an update is pending, as opposed to a screen-erasing obnoxious progress popover). And definitely don’t erase everything the user did if the network ends up being unreachable.