The function EvtBsMuMuKK::Integral() implements trapezoid rule with fixed bin width. One could make the integration better by iterating with decreasing bin width. With the trapezoidal rule, this can be done with the same number of function evaluations as with final bin width. Overall it will not have a large benefit for performance as the integrals are calculated only at initialisation, but would be useful cleanup to avoid others using this as an example of how to do integral. It also evaluates function about twice as many time as necessary.
When I have a bit of time, I will take care of rewriting it.