> I am assuming events cannot finish before they are started, yes! I am pretty sure that even the (in)famous DEC Alpha could not possibly have time-traveling results.

You seem to think this is a joke, but it isn't. Obviously from the CPU's point of view there is no "time travel" but that's cold comfort for users. The Alpha doesn't promise that there's any coherent ordering at all unless you've imposed one, which the sequentially consistent atomics Haiku is apparently asking for do.

To get Sequential Consistency you're paying a very high price on such weakly ordered architectures. On x86 (and x86-64) the relative price is smaller, because everybody is paying for Acquire-release basically all the time, but it's still substantial on larger modern systems.

This price isn't unique to Haiku, but the choice to pay it (almost) everywhere is, at least in terms of operating systems people would be using today.