ILP runtime seems to be linear?
I will try to answer to your question with an example.
Consider the following time complexity: $f(n)=2^\frac{n}{1000}$. 

It is clearly exponential in $n$, but look at the plot.

For $n \in [0,80]$:



From this perspective it looks constant, right?

For $n \in [0, 1.5 \cdot 10^4]$:



From this perspective everything changes.

What I want to show, is that inferring the complexity just looking at the runtime with different inputs size is not possibile.