Dereference a pointer? That translates to a tiny fragment of machine code for "dereference pointer".

Create a lambda? That translates to a big blob of machine code for "set up this bunch of paperwork that will be used to painstakingly emulate a lambda".

Of course, it won't happen magically: someone has to go to the effort of defining the subset and building the compiler for it.

You can't expect the general-purpose compiler to automatically be as optimal as possible just because a program happens to use the subset. For example, the program might be being compiled into something like a java .class file meant for importing to a less conservative calling program. This would be incompatible with putting those bare metal optimizations in the .class file.