A spike for me is pretty much the same as it is for you.
I don't like debating the optimum route thing, because sometimes you may purposely take a sub-optimum route, if it's safer or easier. Sometimes along easy sub-optimum routes, I'll plan several legs in advance, so that's a nice tradeoff. Plus, the difference between routes in these city parks are really just a couple of seconds, a few meters of climb, or a bit extra distance.
So if I quickly choose a reasonable route, and execute it almost perfectly, then I call that a spike. I did what I wanted to do and I knew where I was.
One leg today, I lost about 3 seconds approaching the control because I took the stairs instead of this open area that went right to it. My plan was to run down the stairs the whole way. By the time I saw the open area, I was already past it. Only cost me 3 seconds, so again, not a huge deal, I'll call that a spike (Eric might not).
At O' in the Oaks, I navigated one long leg perfectly. However, I took a horrible, horrible route choice, and lost 5-6 minutes. No spike on that one.