I tried a /castsequence Arcane Blast, Arcane Blast, Arcane Blast, Arcane Blast
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.