diff --git a/hwy/base.h b/hwy/base.h
index ae4f226d2b..764352a443 100644
--- a/hwy/base.h
+++ b/hwy/base.h
@@ -410,8 +410,9 @@ HWY_DLLEXPORT HWY_NORETURN void HWY_FORMAT(3, 4)
 #if !defined(HWY_IS_DEBUG_BUILD)
 // Clang does not define NDEBUG, but it and GCC define __OPTIMIZE__, and recent
 // MSVC defines NDEBUG (if not, could instead check _DEBUG).
-#if (!defined(__OPTIMIZE__) && !defined(NDEBUG)) || \
-    (HWY_IS_SANITIZER && !HWY_IS_UBSAN) || defined(__clang_analyzer__)
+#if (!defined(__OPTIMIZE__) && !defined(NDEBUG)) ||         \
+    (HWY_IS_ASAN || (HWY_IS_SANITIZER && !HWY_IS_UBSAN)) || \
+    defined(__clang_analyzer__)
 #define HWY_IS_DEBUG_BUILD 1
 #else
 #define HWY_IS_DEBUG_BUILD 0
@@ -1746,7 +1747,11 @@ HWY_F16_CONSTEXPR inline std::partial_ordering operator<=>(
 #ifndef HWY_HAVE_SCALAR_BF16_OPERATORS
 // Recent enough compiler also has operators. aarch64 clang 18 hits internal
 // compiler errors on bf16 ToString, hence only enable on GCC for now.
-#if HWY_HAVE_SCALAR_BF16_TYPE && (HWY_COMPILER_GCC_ACTUAL >= 1300)
+// GCC >= 13 will insert a function call to the __extendbfsf2 helper function
+// for scalar conversions from __bf16 to float. This is prohibitively expensive,
+// so refrain from using scalar BF16 operators on these compiler versions.
+// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121853
+#if HWY_HAVE_SCALAR_BF16_TYPE && (HWY_COMPILER_GCC_ACTUAL >= 1700)
 #define HWY_HAVE_SCALAR_BF16_OPERATORS 1
 #else
 #define HWY_HAVE_SCALAR_BF16_OPERATORS 0