The profiler currently issues many fprintf calls and relies on the OS to buffer actual writes to disk. There are also many allocations to take care of file references in profile output files.
In order to improve performance:

• Instead of calling fprintf repeatedly, use xdebug_str to build up one string to fprintf to file
• Use strings for filename/functionname references so that they don't have to be converted from integers to numbers with an sprintf variant
• No longer use xdebug_sprintf to add filename/function name references, but instead use xdebug_str_add_fmt/xdebug_str_add