This is a problem I've had for a while with the keyboard on Mac OS. I've had it on all machines so it's not a hardware thing but when I use the VO combination the computer doesn't always register the action, for example space bar, h for heading and sometimes when I'm navigating it would appear the computer has lost the fact that I'm holding down both the vo keys and mission control pops up. As I say, it only seems to be with combinations of keys and when I type there is no problem.
Has anyone else experienced this? It's more of an annoyance than anything. It's possible that I just tap too quickly but I do especially notice it on web pages where I sometimes have to tap vo space several times or make a point of holding down the combo when attempting to activate a button.
All the best