diff --git a/Makefile b/Makefile
index 9ac82fda6..fe397e018 100644
--- a/Makefile
+++ b/Makefile
@@ -15,8 +15,16 @@ makefiles = \
   tests/local.mk \
   tests/plugins/local.mk
 
-GLOBAL_CXXFLAGS += -g -Wall -include config.h
-
 -include Makefile.config
 
+OPTIMIZE = 1
+
+ifeq ($(OPTIMIZE), 1)
+  GLOBAL_CXXFLAGS += -O3
+else
+  GLOBAL_CXXFLAGS += -O0
+endif
+
 include mk/lib.mk
+
+GLOBAL_CXXFLAGS += -g -Wall -include config.h -std=c++17
diff --git a/configure.ac b/configure.ac
index 8c0f0f26d..9f29a7734 100644
--- a/configure.ac
+++ b/configure.ac
@@ -50,14 +50,11 @@ AC_DEFINE_UNQUOTED(SYSTEM, ["$system"], [platform identifier ('cpu-os')])
 test "$localstatedir" = '${prefix}/var' && localstatedir=/nix/var
 
 
-# Set default flags for nix (as per AC_PROG_CC/CXX docs),
-# while still allowing the user to override them from the command line.
-: ${CFLAGS="-O3"}
-: ${CXXFLAGS="-O3"}
+CFLAGS=
+CXXFLAGS=
 AC_PROG_CC
 AC_PROG_CXX
 AC_PROG_CPP
-AX_CXX_COMPILE_STDCXX_17([noext], [mandatory])
 
 AC_CHECK_TOOL([AR], [ar])
 
diff --git a/flake.nix b/flake.nix
index b3a7ea535..b3a06bdd0 100644
--- a/flake.nix
+++ b/flake.nix
@@ -386,7 +386,6 @@
             pkgs = nixpkgsFor.x86_64-linux;
             officialRelease = false;
           };
-        */
 
         # Check whether we can still evaluate NixOS.
         tests.evalNixOS =
@@ -400,6 +399,7 @@
 
               touch $out
             '';
+        */
 
         # Aggregate job containing the release-critical jobs.
         release =
@@ -421,7 +421,7 @@
                 tests.nix-copy-closure
                 tests.binaryTarball
                 #tests.evalNixpkgs
-                tests.evalNixOS
+                #tests.evalNixOS
                 installerScript
               ];
           };
diff --git a/mk/lib.mk b/mk/lib.mk
index 1da51d879..b0a36cf8d 100644
--- a/mk/lib.mk
+++ b/mk/lib.mk
@@ -89,6 +89,7 @@ include mk/jars.mk
 include mk/patterns.mk
 include mk/templates.mk
 include mk/tests.mk
+include mk/precompiled-headers.mk
 
 
 # Include all sub-Makefiles.
diff --git a/mk/patterns.mk b/mk/patterns.mk
index 3219d9629..7319f4cdd 100644
--- a/mk/patterns.mk
+++ b/mk/patterns.mk
@@ -1,10 +1,10 @@
 $(buildprefix)%.o: %.cc
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(GLOBAL_CXXFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(CXX) -o $@ -c $< $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
 
 $(buildprefix)%.o: %.cpp
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(GLOBAL_CXXFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(CXX) -o $@ -c $< $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
 
 $(buildprefix)%.o: %.c
 	@mkdir -p "$(dir $@)"
diff --git a/mk/precompiled-headers.mk b/mk/precompiled-headers.mk
new file mode 100644
index 000000000..779389b26
--- /dev/null
+++ b/mk/precompiled-headers.mk
@@ -0,0 +1,42 @@
+PRECOMPILE_HEADERS ?= 1
+
+print-var-help += \
+  echo "  PRECOMPILE_HEADERS ($(PRECOMPILE_HEADERS)): Whether to use precompiled headers to speed up the build";
+
+GCH = $(buildprefix)precompiled-headers.h.gch
+
+$(GCH): precompiled-headers.h
+	@rm -f $@
+	@mkdir -p "$(dir $@)"
+	$(trace-gen) $(CXX) -x c++-header -o $@ $^ $(GLOBAL_CXXFLAGS)
+
+PCH = $(buildprefix)precompiled-headers.h.pch
+
+$(PCH): precompiled-headers.h
+	@rm -f $@
+	@mkdir -p "$(dir $@)"
+	$(trace-gen) $(CXX) -x c++-header -o $@ $^ $(GLOBAL_CXXFLAGS)
+
+clean-files += $(GCH) $(PCH)
+
+ifeq ($(PRECOMPILE_HEADERS), 1)
+
+  ifeq ($(CXX), g++)
+
+    GLOBAL_CXXFLAGS_PCH += -include $(buildprefix)precompiled-headers.h -Winvalid-pch
+
+    GLOBAL_ORDER_AFTER += $(GCH)
+
+  else ifeq ($(CXX), clang++)
+
+    GLOBAL_CXXFLAGS_PCH += -include-pch $(PCH) -Winvalid-pch
+
+    GLOBAL_ORDER_AFTER += $(PCH)
+
+  else
+
+    $(error Don't know how to precompile headers on $(CXX))
+
+  endif
+
+endif
diff --git a/perl/Makefile b/perl/Makefile
index f36f5d0e9..284c75022 100644
--- a/perl/Makefile
+++ b/perl/Makefile
@@ -4,4 +4,11 @@ GLOBAL_CXXFLAGS += -g -Wall
 
 -include Makefile.config
 
+OPTIMIZE = 1
+
+ifeq ($(OPTIMIZE), 1)
+  GLOBAL_CFLAGS += -O3
+  GLOBAL_CXXFLAGS += -O3
+endif
+
 include mk/lib.mk
diff --git a/perl/configure.ac b/perl/configure.ac
index e8e3610a8..966700695 100644
--- a/perl/configure.ac
+++ b/perl/configure.ac
@@ -2,10 +2,8 @@ AC_INIT(nix-perl, m4_esyscmd([bash -c "echo -n $(cat ../.version)$VERSION_SUFFIX
 AC_CONFIG_SRCDIR(MANIFEST)
 AC_CONFIG_AUX_DIR(../config)
 
-# Set default flags for nix (as per AC_PROG_CC/CXX docs),
-# while still allowing the user to override them from the command line.
-: ${CFLAGS="-O3"}
-: ${CXXFLAGS="-O3"}
+CFLAGS=
+CXXFLAGS=
 AC_PROG_CC
 AC_PROG_CXX
 AX_CXX_COMPILE_STDCXX_11
diff --git a/precompiled-headers.h b/precompiled-headers.h
new file mode 100644
index 000000000..c5790bec4
--- /dev/null
+++ b/precompiled-headers.h
@@ -0,0 +1,71 @@
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <cassert>
+#include <cctype>
+#include <chrono>
+#include <climits>
+#include <cmath>
+#include <condition_variable>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <exception>
+#include <functional>
+#include <future>
+#include <iostream>
+#include <limits>
+#include <list>
+#include <locale>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <numeric>
+#include <optional>
+#include <queue>
+#include <random>
+#include <regex>
+#include <set>
+#include <sstream>
+#include <stack>
+#include <stdexcept>
+#include <string>
+#include <thread>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include <boost/format.hpp>
+
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <grp.h>
+#include <net/if.h>
+#include <netdb.h>
+#include <netinet/ip.h>
+#include <pwd.h>
+#include <sched.h>
+#include <signal.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/mount.h>
+#include <sys/param.h>
+#include <sys/personality.h>
+#include <sys/resource.h>
+#include <sys/select.h>
+#include <sys/socket.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <sys/utsname.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <unistd.h>
+
+#include "util.hh"
+#include "args.hh"
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index fe861e629..bf259e0b6 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -24,8 +24,8 @@
 #include <chrono>
 #include <regex>
 #include <queue>
+#include <climits>
 
-#include <limits.h>
 #include <sys/time.h>
 #include <sys/wait.h>
 #include <sys/types.h>
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index a5165773a..d49d63912 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -34,6 +34,10 @@ DownloadSettings downloadSettings;
 
 static GlobalConfig::Register r1(&downloadSettings);
 
+CachedDownloadRequest::CachedDownloadRequest(const std::string & uri)
+    : uri(uri), ttl(settings.tarballTtl)
+{ }
+
 std::string resolveUri(const std::string & uri)
 {
     if (uri.compare(0, 8, "channel:") == 0)
diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index c5dd893b5..487036833 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -2,7 +2,7 @@
 
 #include "types.hh"
 #include "hash.hh"
-#include "globals.hh"
+#include "config.hh"
 
 #include <string>
 #include <future>
@@ -71,12 +71,12 @@ struct CachedDownloadRequest
     bool unpack = false;
     std::string name;
     Hash expectedHash;
-    unsigned int ttl = settings.tarballTtl;
+    unsigned int ttl;
     bool gcRoot = false;
     bool getLastModified = false;
 
-    CachedDownloadRequest(const std::string & uri)
-        : uri(uri) { }
+    CachedDownloadRequest(const std::string & uri);
+    CachedDownloadRequest() = delete;
 };
 
 struct CachedDownloadResult
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 0902c3783..998037337 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -10,6 +10,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <climits>
 #include <iostream>
 #include <sstream>
 #include <thread>
@@ -17,7 +18,6 @@
 
 #include <fcntl.h>
 #include <grp.h>
-#include <limits.h>
 #include <pwd.h>
 #include <sys/ioctl.h>
 #include <sys/types.h>
diff --git a/src/libutil/xml-writer.cc b/src/libutil/xml-writer.cc
index e5cc2e9fc..68857e34d 100644
--- a/src/libutil/xml-writer.cc
+++ b/src/libutil/xml-writer.cc
@@ -1,10 +1,10 @@
-#include <assert.h>
+#include <cassert>
 
 #include "xml-writer.hh"
 
 
 namespace nix {
-    
+
 
 XMLWriter::XMLWriter(bool indent, std::ostream & output)
     : output(output), indent(indent)
diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index da17ddcba..65ae88faf 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -10,8 +10,9 @@
 #include "daemon.hh"
 
 #include <algorithm>
-
+#include <climits>
 #include <cstring>
+
 #include <unistd.h>
 #include <signal.h>
 #include <sys/types.h>
@@ -23,7 +24,6 @@
 #include <pwd.h>
 #include <grp.h>
 #include <fcntl.h>
-#include <limits.h>
 
 #if __APPLE__ || __FreeBSD__
 #include <sys/ucred.h>