If the dictionary's assessment that too simply emphasizes the statement, you could interpret it in the following way. Keep in mind that the end result is the same: too simply emphasizes the preceding statement!
There are plenty of things I can say to describe my feeling about the fact that she gave you the money: that she even thought about not giving it to you, or the that you had to ask for it, etc.
On top of all that there is another thing, that it took her so long.
By leaving out the other things I could say about it, I emphasize the part that I also want to mention. If I mention all of them, it could go like this:
She gave me the money.
Ridiculous you even had to ask for it! That she even considered not giving you the money! And also, it's about time she paid you!
Leaving out the other points, I emphasize my point about the delay, and I insinuate there are other things I could say about it:
About time, too!