>Big O

Yup, I get that, that's why the system will spit out "hello world" in like 0.03 seconds, but take 0.22 to produce the 400 line output for the forum sections page for instance. More data = more computing = slower result.

I'm afraid you're not getting it at all.

Complexity is not about "there's more input, so it will take longer", it's the indication how it will scale. Does it scale lineary, quadratic, logarithmic, etc.