Note that QwtSplineCurveFitter is only a wrapper around the actual QwtSpline classes, that you don't need in your case.

The default implementation of QwtSplineCurveFitter uses a parametric cardinal spline with a uniform parametrization. In case you have specific requirements ( like performance ) it might be worth to find the right spline interpolation/approximation options for you. F.e when your curve has increasing x or y values you don't need parametrization and could go with ParameterX/ParameterY ( = no parametrization ) instead.

( Parametrization is always doing things twice: x = f(t) and y = g(t) instead of y = f(x) )

Uwe