As Xdebug treats include and friends as actual function calls, I can unfortunately not solely rely on the Observer API. Another optimisation that I worked on was to optimise the way how function breakpoints are checked against. Xdebug would evaluate whether an IDE has set a line breakpoint on the current line. If no breakpoints are set in the whole function, then Xdebug skips the check for line breakpoints after each statement. This shifts the factor of performance loss for having line breakpoints from the number of statements to the number of function calls.