diff --git a/platformio.ini b/platformio.ini
index 9fa302349..73646abfb 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -136,6 +136,20 @@ board = s_16M_P
 extends = espressif32_base_16M
 board = s3_16M_P
 
+[env:s3_16M_P_custom]
+; same as s3_16M_P, but the Arduino/IDF libs are rebuilt with HybridCompile using the
+; custom_sdkconfig below, to compare flash size and free heap against stock s3_16M_P.
+; first build takes 10-20 min (IDF compiled from source), later builds are cached.
+; building s3_16M_P and this env alternately triggers a rebuild each time.
+extends = env:s3_16M_P
+custom_sdkconfig =
+  '# CONFIG_BT_ENABLED is not set'
+  '# CONFIG_LWIP_PPP_SUPPORT is not set'
+  '# CONFIG_LWIP_IP_FORWARD is not set'
+  '# CONFIG_LWIP_IPV4_NAPT is not set'
+  '# CONFIG_ULP_COPROC_ENABLED is not set'
+  CONFIG_LWIP_LOCAL_HOSTNAME="ems-esp"
+
 [env:s3_32M_P]
 ; 16MB ESP32-S3 - with PSRAM and 32MB flash
 extends = espressif32_base_32M