and I noticed it always produces a slight delay before each AB. Using AB on its own does not have it.
I also confirmed it on another spec and another spell.
It appears to be around 200ms. Much lower than the GCD but very noticeable. The number is a guess.
PS. The example sequence has no real use but I first noticed it on a sequence that included Mirror Image. At first I suspected an issue with MI, but removing it did not alter the latency effect.
EDIT: I suspect it has a simple explanation: The cast buffer may not be taken into account at all hence, to produce latency equivalent to world latency.