Eth iperf default sdkconfig: Enable lwIP IRAM optimization

examples/ethernet/iperf/README.md

@@ -146,7 +146,7 @@ I (2534456) iperf: want recv=16384
 ## Suggestions of getting higher bandwidth
 
 1. Higher MCU working frequency will get higher bandwidth.
-2. Put frequency invoked function into IRAM via macro `IRAM_ATTR` in code.
+2. Put frequently invoked functions into IRAM via macro `IRAM_ATTR` in code. Note that the lwIP IRAM optimization is already enabled by default.
 3. Priority of iperf task may also have effect.
 
 ## Troubleshooting

examples/ethernet/iperf/sdkconfig.defaults

@@ -18,3 +18,6 @@ CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
 # Disable watch dog
 CONFIG_ESP_INT_WDT=n
 CONFIG_ESP_TASK_WDT=n
+
+# Enable lwIP IRAM optimization
+CONFIG_LWIP_IRAM_OPTIMIZATION=y