Evidence that Macros Do NOT Cause GCD Delay or Lost Casts

[Rin_Sato]

I'm genuinely unsure what you think you're demonstrating here. By mashing two macros alternately, each macro has the potential to interrupt both itself and the other macro; that's part of why in my first post I clearly recommend that macros not be mashed. Don't do that and you'll be fine.

As an aside, I've heard from others that /echo has some interesting timing behavior that differs from other macro commands such as /ac, though I haven't looked into it myself since I don't use /echo macros. I can't tell if that's part of what you're seeing since the mashing is obviously going to get in the way.

What is being demonstrated is that each line in a macro adds a delay.
By exacerbating the issue through adding duplicate lines the issue as well as my point is made more clear: macros add an inherent delay independent of GCD.
It doesn't take much reasoning past that to infer that they are (to an extent) going to cause a loss in GCD.