Actually I've found oGCD macros do slow down your wheel, you can hit 1 oGCD macro'd without delaying the GCD wheel, but if you try hitting 2 oGCDs (that are both macro) it does.

for example a macro of:

/ac Sentinel <me>
/ac Bloodbath <me>

Hitting it repeatedly between a GCD will delay the next. Also I've felt you need to tap it repeatedly - as the click doesn't 'register' unless the GCD wheel is passed 30-40%

without the macro just using the 2 oGCDs vanilla will be faster and not delay the next GCD (although Bloodbath may be a bad example, I feel that that OGCD has relatively long animation - but most others can do 2 oGCD between GCD without interrupting)