Help. Double-clicking on my Mac is unreliable.

The trouble started when I had to do a fresh install of Mac OS a couple of months ago.

I find that when I double-click the first time, all is good. But the second time, it doesn't register.

If I double-click, move the mouse, and double-click -- it works.

I know how to get to the Accessibility panel to set the Double-click speed. I've been changing it all the time for the last two months.

It's finally time to do something about it. Before the OS reinstall it worked flawlessly. I never had to think about it.

Same OS version btw. 10.10.1 (Yosemite)

My first question is this -- what does Double-click speed mean? I've searched, haven't been able to find a good definition. The docs on the net about this are very confusing.