diff --git a/board/cm-bf537e/config.mk b/board/cm-bf537e/config.mk
index c0cb9ff..7d1fc44 100644
--- a/board/cm-bf537e/config.mk
+++ b/board/cm-bf537e/config.mk
@@ -25,6 +25,7 @@
 
 CFLAGS_lib += -O2
 CFLAGS_lib/lzma += -O2
+CFLAGS_lib/zlib += -O2
 
 # Set some default LDR flags based on boot mode.
 LDR_FLAGS-BFIN_BOOT_PARA := --bits 16 --dma 8