1. The game does not accept decimals for /wait. When you type /wait 2.5 the game rounds this number up to 3 which is why it seems so slow to you. When you type /wait 2.4 the game rounds down to 2 and because macros cannot be queued up you get the "too soon" error. Because of this putting multiple on-GCD abilities into a macro will lower your DPS.

2. The game has the one of the worst gameplay mechanics ever: Animation Lag. Even if you use an ability off the GCD you cannot use another ability immediately afterwards (some abilities may not have enough animation lag to be noticeable but others most definitely do). For example, a BLM macro like this:

/ac "Swiftcast" <me>
/ac "Raging Strikes" <me>
/ac Fire III

will not work with one key press. That being said, spamming this macro will probably be slightly faster than spamming each separate ability so there is a small advantage to a macro like this.