diff --git a/Makefile b/Makefile
index 52df845..60bd00b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -Wall -Werror -pedantic -g
+CFLAGS = -Wall -Werror -pedantic -O3
 
 .PHONY: all
 all: um
diff --git a/README.org b/README.org
index 117d497..a96c5e9 100644
--- a/README.org
+++ b/README.org
@@ -14,6 +14,15 @@ and run it on a UM program:
   ./um codex.umz
 #+end_src
 
+Benchmark times:
+#+begin_src sh
+# time ./um sandmark.umz
+# without optimization
+./um sandmark.umz  86.20s user 0.00s system 99% cpu 1:26.20 total
+# with -O3
+./um sandmark.umz  30.06s user 0.00s system 99% cpu 30.065 total
+#+end_src
+
 Get the next program:
 
 #+begin_src sh