The 2 missing pieces of the puzzle here is the internal global cooldown, or the iGCD, and macro queuing.

The game has an innate cool down on ALL abilities, which is roughly 1.33 seconds. What this means, is that if you are trying to use 2 off-GCD skills between GCDs, you are burning 3.99s (GCD Skill > 1.33s > oGCD Skill > 1.33s > oGCD Skill > 1.33s), which is going to interfere with getting your next GCD off, as you will be well over the average 2.8ish seconds between GCD skills. If you only use one skill between GCDs, you should be golden, only using 2.66s between.

Second, when you macro an ability, it does NOT queue the ability. What I mean by this is that if you have a macro that is "/ac Heavy Swing <t>", you will need to wait for your iGCD or GCD to completely expire before the game will register you having activated that button. AS OPPOSED to a normal hot-keyed action of Heavy Swing, which the game will recognize you have pressed roughly .3 seconds before the GCD or iGCD expires, meaning it will fire off at the exact second it is available as long as you activate it within .3 seconds before the cool down is up, instead of relying on your reflexes of watching the cool-down expire. The computer will ALWAYS win. So if you are waiting on macros, you are definitely losing .05-.3 seconds per skill, which adds up A LOT over time.

Hope that helped.