I would suggest a slightly different virtual array implementation. Currently this requires calculating the length of all splines, regardless of the IndexMask. A virtual array that only calculated the necessary elements would be helpful here, since each calculation isn't trivial if the result has not already been cached on the spline:

Something like this should work. Though the constructors are a bit complex, it's also relatively simple to read.

Span<SplinePtr> splines = curve.splines();
auto length_fn = [splines](int i) { return splines[i]->length(); };
return &scope.construct<
    fn::GVArray_For_EmbeddedVArray<float, VArray_For_Func<float, decltype(length_fn)>>>(
    splines.size(), splines.size(), length_fn);

This should also handle the point domain. In that case, it can use the curve component's attribute_try_adapt_domain method.