cores/arduino/HardwareSerial.cpp

@@ -140,7 +140,7 @@ void HardwareSerial::rx_complete_irq(void)
 }
 
 // wrapper for use in С-files
-extern "C" void serial_interrupt_handler(uint8_t uartNumInt) 
+extern "C" void __attribute__((optimize("O3"))) serial_interrupt_handler(uint8_t uartNumInt) 
 {
   if (uartNumInt == 0)
   {

cores/arduino/HardwareSerial.h

@@ -100,7 +100,7 @@ class HardwareSerial : public Stream
     using Print::write; // pull in write(str)
     operator bool() { return isInited; }
 
-    void rx_complete_irq(void);
+    void rx_complete_irq(void) __attribute__((optimize("O3")));
 };
 
 extern HardwareSerial Serial;

cores/arduino/WInterrupts.c

@@ -114,7 +114,7 @@ void enableInterrupt(uint8_t interruptNum)
 }
 
 // common gpio interrupt handler 
-void gpio_interrupt_handler(void) __attribute__((noinline, section(".ram_text")));
+void gpio_interrupt_handler(void) __attribute__((noinline, section(".ram_text"), optimize("O3")));
 void gpio_interrupt_handler(void)
 {
   // go through all the interrupts and call the handler for the triggered line