MPB runtime vs. number of k-vectors?

Hi MPB community,

by chance I noticed that the runtime of MPB is not linear with the number 
of k-vectors to compute. So I did a small runtime study for different 
numbers of k-vectors. I used the 1D bragg example for that, specifying the 
number of k-vectors by changing the numberofkvectors in the command 
"(interpolate numberofkvectors k-points)". I plotted numberofkvectors vs. 
the runtime (from the "total elapsed time for run" at the end of the 
out-file) and it can be nicely fit with a quadratic function:

numberofkvectors, runtime [seconds]
5000, 1
10000, 4
20000, 13
50000, 73
100000, 279
200000, 1140
500000, 7297

I would have expected a linear O(n) dependency assuming that each k-point 
needs approximately the same effort to compute its solution. Given a 
non-linear runtime, it would make sense to split simulations into several 
runs with less k-vectors in each simulation...
Am I missing something? Or is this just a special case with that setup?

Thanks in advance!
Johannes

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