-O2 is the same speed but smaller size

platformio.ini

@@ -29,7 +29,7 @@ build_flags =
     -DARDUINO=10813
     -DARDUINO_ARCH_STM32
     -DDEBUG_LEVEL=DEBUG_NONE
-    -O3
+    -O2
 
 ; Work around for clones.
 ; https://community.platformio.org/t/cannot-upload-to-stm32-bluepill-board-over-stlink-v2/3492/25