When the step debugger is active it always instructs the framework to collect information about local variables (ie. compiled variables). This adds unnecessary overhead.

Secondly, Xdebug checks too often for breakpoints:

• for function breakpoints, even if there are none it will do work to build a string representation of the name
• for line breakpoints, at every statement against all line breakpoints, even if we already can establish that a function does not have any line breakpoints.

Add optimisations to address all these three issues.