Use "real" full optimization flag with MSVC (/O2 instead of /Ox)

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build.rs

@@ -619,7 +619,10 @@ fn cc(file: &Path, ext: &str, target: &Target, include_dir: &Path, out_file: &Pa
                                    // run-time checking: (s)tack frame, (u)ninitialized variables
             let _ = c.flag("/RTCsu");
         } else {
-            let _ = c.flag("/Ox"); // Enable full optimization.
+            // Enable full optimization.
+            // Despite popular belief, /Ox enables less optimizations than /O2
+            // https://learn.microsoft.com/en-us/cpp/build/reference/ox-full-optimization?view=msvc-170
+            let _ = c.flag("/O2");
         }
     }