The deciding factor is the number of VST preset slots the plugin supports.
If the plugin supports multiple preset slots (eg: Ivory) then Cantabile works directly with those and exports as FXB files.
If the plugin only supports one preset slot (eg: Omnisphere) then Cantabile switches to “pseudo preset mode” where it maintains a copy of the entire plugin state for each preset slot. Under the covers Cantabile maintains an FXB for each preset and on selecting the preset loads that internally managed FXB.
When you export a bank from a pseudo preset managed plugin, there needs to be a way to export all these internally managed FXB’s as one file - and that’s what a .cantabileFxb file is - a special format only supported by Cantabile that contains multiple FXB files.
Regarding load times, switching presets on a non-pseudo plugin Cantabile tells the plugin “load preset N”. When switching pseudo presets, Cantabile says “here load this FXB”. How long the plugin takes to do that can vary wildly.