Update README for clarity on performance comparison

Thanks to Matthias Kretz for the idea.

README.md

@@ -150,8 +150,8 @@ int main() {
 }
 ```
 
-This can be [5 to 9 times faster than
-fprintf](http://www.zverovich.net/2020/08/04/optimal-file-buffer-size.html).
+This can be [up to 9 times faster than `fprintf`](
+http://www.zverovich.net/2020/08/04/optimal-file-buffer-size.html).
 
 **Print with colors and text styles**
 

include/fmt/format-inl.h

@@ -30,6 +30,14 @@
 #  define FMT_FUNC
 #endif
 
+#if defined(FMT_USE_FULL_CACHE_DRAGONBOX)
+// Use the provided definition.
+#elif defined(__OPTIMIZE_SIZE__)
+#  define FMT_USE_FULL_CACHE_DRAGONBOX 0
+#else
+#  define FMT_USE_FULL_CACHE_DRAGONBOX 1
+#endif
+
 FMT_BEGIN_NAMESPACE
 
 #ifndef FMT_CUSTOM_ASSERT_FAIL

include/fmt/format.h

@@ -736,10 +736,6 @@ using fast_float_t = conditional_t<sizeof(T) == sizeof(double), double, float>;
 template <typename T>
 using is_double_double = bool_constant<std::numeric_limits<T>::digits == 106>;
 
-#ifndef FMT_USE_FULL_CACHE_DRAGONBOX
-#  define FMT_USE_FULL_CACHE_DRAGONBOX 0
-#endif
-
 // An allocator that uses malloc/free to allow removing dependency on the C++
 // standard libary runtime. std::decay is used for back_inserter to be found by
 // ADL when applied to memory_buffer.