Elaborating on what James said, an excerpt from my Mass notes:
In general, the Mass StateTree is limited to the Game thread, however it does NOT necessarily tick on every frame.
The Mass StateTree ticks for a given Entity only when it is explicitly signaled by UMassStateTreeProcessor, which itself listens for and reacts to specific UMassSignalSubsystem signals.
Thus, in your C++ code you must explicitly manage the ticking of the StateTree and ensure to signal Mass every time you want a given StateTree to tick.
Due to this CPU-efficient ticking policy, in your StateTree Task Tick methods, you will see VERY HIGH values for DeltaTime. The DeltaTime in this case is not the game thread DeltaTime, it is instead the full length of time that has elapsed since the last explicit StateTree tick, which could be many seconds ago (or longer).
For more info RE Mass StateTree (or Mass in general) I’m keeping notes as I learn about it: