Add a comment about faster compilation

Tools/jit/_targets.py

@@ -195,6 +195,11 @@ async def _build_stencils(self) -> dict[str, _stencils.StencilGroup]:
                 tasks.append(group.create_task(coro, name="trampoline"))
                 template = TOOLS_JIT_TEMPLATE_C.read_text()
                 for case, opname in cases_and_opnames:
+                    # Write out a copy of the template with *only* this case
+                    # inserted. This is about twice as fast as #include'ing all
+                    # of executor_cases.c.h each time we compile (since the C
+                    # compiler wastes a bunch of time parsing the dead code for
+                    # all of the other cases):
                     c = work / f"{opname}.c"
                     c.write_text(template.replace("CASE", case))
                     coro = self._compile(opname, c, work)