MSVC massively scales down on optimization depending of the function size.
bison generated MySQLParse() is considered "huge" by the compiler . I verified this by adding /we4883 flag to compiler options. That makes making (usually inactive) warning "function size prevents optimization" an error.
There is a switch that allows optimizations also for such functions, /d2OptimizeHugeFunctions.
MySQL 8.0 is using it, according to