diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 07b540270..44e5961a1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -29,11 +29,13 @@ make devsetup
 
 ### Tips and Tricks
 
-- Build the code: `make -j`.
+- Build the code faster: `make -j BUILD_MODE=default`. By default `make` builds infer with flambda
+  enabled, which makes it very slow (but makes infer significantly faster).
 
 - Faster edit/build cycle when working on OCaml code inside infer/src/: build inside infer/src/
   (skips building the models after infer has been built), and build bytecode instead of native:
-  `make -j -C infer/src byte`. You need to have run `make -j` at some point before.
+  `make -j -C infer/src byte`. You need to have run `make -j` (with or without `BUILD_MODE=default`)
+  at some point before.
 
 - In general, `make` commands from the root of the repository make sure that dependencies are in a
   consistent and up-to-date state (e.g., they rebuild infer and the models before running steps that
@@ -44,6 +46,9 @@ make devsetup
   before running the test, but running `make -C infer/tests/codetoanalyze/java/infer test` will just
   execute the test.
 
+- To switch the default build mode to flambda disabled, you can `export BUILD_MODE=default` in your
+  shell.
+
 ## Hacking on the Code in facebook-clang-plugins 
 
 Infer uses `ASTExporter` from the [facebook-clang-plugins](https://github.com/facebook/facebook-clang-plugins)
diff --git a/Makefile b/Makefile
index 2fe453219..64743bce7 100644
--- a/Makefile
+++ b/Makefile
@@ -11,6 +11,11 @@ default: infer
 ROOT_DIR = .
 include $(ROOT_DIR)/Makefile.config
 
+# override this for faster builds (but slower infer)
+BUILD_MODE ?= opt
+
+MAKE_SOURCE = $(MAKE) -C $(SRC_DIR) INFER_BUILD_DIR=_build/$(BUILD_MODE)
+
 ifneq ($(UTOP),no)
 BUILD_SYSTEMS_TESTS += infertop
 build_infertop_print build_infertop_test: test_build
@@ -147,22 +152,22 @@ fmt_all:
 .PHONY: src_build_common
 src_build_common:
 	$(QUIET)$(call silent_on_success,Generating source dependencies,\
-	$(MAKE) -C $(SRC_DIR) src_build_common)
+	$(MAKE_SOURCE) src_build_common)
 
 .PHONY: src_build
 src_build: src_build_common
 	$(QUIET)$(call silent_on_success,Building native Infer,\
-	$(MAKE) -C $(SRC_DIR) infer)
+	$(MAKE_SOURCE) infer)
 
 .PHONY: byte
 byte: src_build_common
 	$(QUIET)$(call silent_on_success,Building byte Infer,\
-	$(MAKE) -C $(SRC_DIR) byte)
+	$(MAKE_SOURCE) byte)
 
 .PHONY: test_build
 test_build: src_build_common
 	$(QUIET)$(call silent_on_success,Testing Infer builds without warnings,\
-	$(MAKE) -C $(SRC_DIR) test)
+	$(MAKE_SOURCE) test)
 
 ifeq ($(IS_FACEBOOK_TREE),yes)
 byte src_build_common src_build test_build: fb-setup
@@ -608,7 +613,17 @@ devsetup: Makefile.autoconf
 	  if [ "$$infer_repo_is_in_manpath" != "0" ]; then \
 	    printf "$(TERM_INFO)  echo 'export MANPATH=\"%s/infer/man\":\$$MANPATH' >> \"$$shell_config_file\"$(TERM_RESET)\n" "$(ABSOLUTE_ROOT_DIR)" >&2; \
 	  fi; \
-	fi
+	fi; \
+	test "$$BUILD_MODE" = "default" || \
+	  echo >&2; \
+	  echo '$(TERM_INFO)*** NOTE: Set `BUILD_MODE=default` in your shell to disable flambda by default.$(TERM_RESET)' >&2; \
+	  echo '$(TERM_INFO)*** NOTE: Compiling with flambda is ~5 times slower than without, so unless you are$(TERM_RESET)' >&2; \
+	  echo '$(TERM_INFO)*** NOTE: testing infer on a very large project it will not be worth it. Use the$(TERM_RESET)' >&2; \
+	  echo '$(TERM_INFO)*** NOTE: commands below to set the default build mode. You can then use `make BUILD_MODE=opt`$(TERM_RESET)' >&2; \
+	  echo '$(TERM_INFO)*** NOTE: when you really do want to enable flambda.$(TERM_RESET)' >&2; \
+	  echo >&2; \
+	  printf "$(TERM_INFO)  export BUILD_MODE=default$(TERM_RESET)\n" >&2; \
+	  printf "$(TERM_INFO)  echo 'export BUILD_MODE=default' >> \"$$shell_config_file\"$(TERM_RESET)\n" >&2
 	$(QUIET)PATH=$(ORIG_SHELL_PATH); if [ "$$(ocamlc -where 2>/dev/null)" != "$$($(OCAMLC) -where)" ]; then \
 	  echo >&2; \
 	  echo '$(TERM_INFO)*** NOTE: The current shell is not set up for the right opam switch.$(TERM_RESET)' >&2; \
diff --git a/build-infer.sh b/build-infer.sh
index 8581b31e4..8a2097380 100755
--- a/build-infer.sh
+++ b/build-infer.sh
@@ -16,9 +16,8 @@ INFER_ROOT="$SCRIPT_DIR"
 INFER_DEPS_DIR="$INFER_ROOT/dependencies/infer-deps"
 PLATFORM="$(uname)"
 NCPU="$(getconf _NPROCESSORS_ONLN 2>/dev/null || echo 1)"
-OCAML_VERSION=${OCAML_VERSION:-"4.05.0"}
+OCAML_VERSION=${OCAML_VERSION:-"4.05.0+flambda"}
 OPAM_LOCK_URL=${OPAM_LOCK_URL:-"https://github.com/rgrinberg/opam-lock"}
-
 INFER_OPAM_SWITCH_DEFAULT=infer-"$OCAML_VERSION"
 
 function usage() {
diff --git a/infer/src/Makefile b/infer/src/Makefile
index aff0b563e..e895d0d5f 100644
--- a/infer/src/Makefile
+++ b/infer/src/Makefile
@@ -12,6 +12,7 @@ include $(ROOT_DIR)/Makefile.config
 
 ETC_DIR = $(INFER_DIR)/etc
 # paths to BUILD_DIR are relative because that's how jbuilder likes it
+# can be overriden to specify another build mode (eg opt)
 INFER_BUILD_DIR = _build/default
 
 ATDGEN_SUFFIXES = _t.ml _t.mli _j.ml _j.mli
diff --git a/infer/src/atd/jbuild.in b/infer/src/atd/jbuild.in
index f640f7a59..1aa5e2314 100644
--- a/infer/src/atd/jbuild.in
+++ b/infer/src/atd/jbuild.in
@@ -3,12 +3,14 @@
 
 let cflags = common_cflags @ ["-w"; "-27-32-34-35-39"]
 
-;; Format.sprintf {|
+;; Format.sprintf
+     {|
 (library
  ((name InferGenerated)
   (flags (%s))
+  (ocamlopt_flags (%s))
   (libraries (atdgen))
  ))
 |}
-     (String.concat " " cflags)
+     (String.concat " " cflags) (String.concat " " common_optflags)
    |> Jbuild_plugin.V1.send
diff --git a/infer/src/istd/jbuild.in b/infer/src/istd/jbuild.in
index 4300d4497..4c140144c 100644
--- a/infer/src/istd/jbuild.in
+++ b/infer/src/istd/jbuild.in
@@ -1,12 +1,14 @@
-(* -*- tuareg -*- *)
-(* NOTE: prepend jbuild.common to this file! *)
-
-;; Format.sprintf {|
+;; (* -*- tuareg -*- *)
+   (* NOTE: prepend jbuild.common to this file! *)
+   Format.sprintf
+     {|
 (library
  ((name InferStdlib)
   (flags (%s))
+  (ocamlopt_flags (%s))
   (libraries (%s))
  ))
 |}
-     (String.concat " " common_cflags) (String.concat " " common_libraries)
+     (String.concat " " common_cflags) (String.concat " " common_optflags)
+     (String.concat " " common_libraries)
    |> Jbuild_plugin.V1.send
diff --git a/infer/src/jbuild-workspace.in b/infer/src/jbuild-workspace.in
index bb5c3ebfa..a4d347424 100644
--- a/infer/src/jbuild-workspace.in
+++ b/infer/src/jbuild-workspace.in
@@ -6,4 +6,5 @@
 ;; files.
 
 (context ((switch @OPAMSWITCH@) (name default)))
+(context ((switch @OPAMSWITCH@) (name opt)))
 (context ((switch @OPAMSWITCH@) (name test)))
diff --git a/infer/src/jbuild.common.in b/infer/src/jbuild.common.in
index 2d79e59c4..f2e39c49a 100644
--- a/infer/src/jbuild.common.in
+++ b/infer/src/jbuild.common.in
@@ -1,6 +1,19 @@
 (* -*- tuareg -*- *)
 (* use strings so that it looks like OCaml even before substituting, e.g. to use ocamlformat *)
 
+type build_mode = Default | Opt | Test
+
+let build_mode =
+  match Jbuild_plugin.V1.context with
+  | "test"
+   -> Test
+  | "default"
+   -> Default
+  | "opt"
+   -> Opt
+  | ctx
+   -> invalid_arg ("unknown context: " ^ ctx)
+
 let is_yes = String.equal "yes"
 
 let clang = is_yes "@BUILD_C_ANALYZERS@"
@@ -27,13 +40,15 @@ let common_cflags =
     ; "-w"
     ; warnings ]
   in
-  match Jbuild_plugin.V1.context with
-  | "test"
-   -> "-warn-error" :: fatal_warnings :: common_flags
-  | "default"
+  match build_mode with
+  | Default | Opt
    -> common_flags
-  | ctx
-   -> invalid_arg ("unknown context: " ^ ctx)
+  | Test
+   -> "-warn-error" :: fatal_warnings :: common_flags
+
+let common_optflags = match build_mode with
+  | Opt -> ["-O3"]
+  | Default | Test -> []
 
 let common_libraries =
   (if java then ["javalib"; "ptrees"; "sawja"] else [])
diff --git a/infer/src/jbuild.in b/infer/src/jbuild.in
index f24cc418d..2191ef2d9 100644
--- a/infer/src/jbuild.in
+++ b/infer/src/jbuild.in
@@ -74,35 +74,39 @@ let stanzas =
 (library
  ((name InferModules)
   (flags (%s))
+  (ocamlopt_flags (%s))
   (libraries (%s))
   (modules (:standard \ %s infertop))
   (preprocess (pps (ppx_compare ppx_sexp_conv -no-check)))
  ))
 |}
-        (String.concat " " infer_cflags) (String.concat " " infer_libraries)
-        (String.concat " " infer_binaries)
+        (String.concat " " infer_cflags) (String.concat " " common_optflags)
+        (String.concat " " infer_libraries) (String.concat " " infer_binaries)
     ; Format.sprintf
         {|
 (executables
  ((names (%s))
   (flags (%s -open InferModules))
+  (ocamlopt_flags (%s))
   (libraries (InferModules))
   (modules (%s))
   (preprocess (pps (ppx_compare ppx_sexp_conv -no-check)))
  ))
 |}
         (String.concat " " infer_binaries) (String.concat " " infer_cflags)
-        (String.concat " " infer_binaries)
+        (String.concat " " common_optflags) (String.concat " " infer_binaries)
     ; Format.sprintf
         {|
 (executable
  ((name infertop)
   (flags (%s))
+  (ocamlopt_flags (%s))
   (libraries (utop InferModules))
   (modules (:standard \ %s))
   (link_flags (-linkall -warn-error -31))))
 |}
-        (String.concat " " infer_cflags) (String.concat " " infer_binaries) ]
+        (String.concat " " infer_cflags) (String.concat " " common_optflags)
+        (String.concat " " infer_binaries) ]
   @ List.map
       (fun source ->
         Printf.sprintf "(rule (%s %s %s))" (copy_action_of_source source) source