diff --git a/src/libutil/fmt.cc b/src/libutil/fmt.cc
new file mode 100644
index 000000000..400fb7ea0
--- /dev/null
+++ b/src/libutil/fmt.cc
@@ -0,0 +1,14 @@
+#include "fmt.hh" // IWYU pragma: keep
+
+template class boost::basic_format<char>;
+
+namespace nix {
+
+// Explicit instantiation saves about 30 cpu-seconds of compile time
+template HintFmt::HintFmt(const std::string &, const Uncolored<std::string> &s);
+template HintFmt::HintFmt(const std::string &, const std::string &s);
+template HintFmt::HintFmt(const std::string &, const uint64_t &, const char * const &);
+
+HintFmt::HintFmt(const std::string & literal) : HintFmt("%s", Uncolored(literal)) {}
+
+}
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index d015f7e5f..7589e51e2 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -3,7 +3,6 @@
 
 #include <iostream>
 #include <string>
-#include <optional>
 #include <boost/format.hpp>
 // Darwin and FreeBSD stdenv do not define _GNU_SOURCE but do have _Unwind_Backtrace.
 #if __APPLE__ || __FreeBSD__
@@ -12,6 +11,9 @@
 #include <boost/stacktrace.hpp>
 #include "ansicolor.hh"
 
+// Explicit instantiation in fmt.cc
+extern template class boost::basic_format<char>;
+
 namespace nix {
 
 /**
@@ -157,7 +159,9 @@ public:
      * Format the given string literally, without interpolating format
      * placeholders.
      */
-    HintFmt(const std::string & literal) : HintFmt("%s", Uncolored(literal)) {}
+    // Moved out of line because it was instantiating the template below in
+    // every file in the project.
+    HintFmt(const std::string & literal);
 
     /**
      * Interpolate the given arguments into the format string.
@@ -193,6 +197,11 @@ public:
     }
 };
 
+// Explicit instantiations in fmt.cc
+extern template HintFmt::HintFmt(const std::string &, const Uncolored<std::string> &s);
+extern template HintFmt::HintFmt(const std::string &, const std::string &s);
+extern template HintFmt::HintFmt(const std::string &, const uint64_t &, const char * const &);
+
 std::ostream & operator<<(std::ostream & os, const HintFmt & hf);
 
 }
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 7c0e45e4b..e7f986363 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -17,6 +17,7 @@ libutil_sources = files(
   'experimental-features.cc',
   'file-descriptor.cc',
   'file-system.cc',
+  'fmt.cc',
   'git.cc',
   'hash.cc',
   'hilite.cc',
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index a6dd7e200..4eda1b7e7 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -9,6 +9,31 @@
 
 namespace nix {
 
+template<typename T>
+T readNum(Source & source)
+{
+    unsigned char buf[8];
+    source(charptr_cast<char *>(buf), sizeof(buf));
+
+    auto n = readLittleEndian<uint64_t>(buf);
+
+    if (n > (uint64_t) std::numeric_limits<T>::max())
+        throw SerialisationError("serialised integer %d is too large for type '%s'", n, typeid(T).name());
+
+    return (T) n;
+}
+
+template bool readNum<bool>(Source & source);
+
+template unsigned char readNum<unsigned char>(Source & source);
+
+template unsigned int readNum<unsigned int>(Source & source);
+
+template unsigned long readNum<unsigned long>(Source & source);
+template long readNum<long>(Source & source);
+
+template unsigned long long readNum<unsigned long long>(Source & source);
+template long long readNum<long long>(Source & source);
 
 void BufferedSink::operator () (std::string_view data)
 {
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index d6a22b3e9..9ad8018d0 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -450,19 +450,7 @@ inline Sink & operator<<(Sink & sink, const Error & ex)
 MakeError(SerialisationError, Error);
 
 template<typename T>
-T readNum(Source & source)
-{
-    unsigned char buf[8];
-    source(charptr_cast<char *>(buf), sizeof(buf));
-
-    auto n = readLittleEndian<uint64_t>(buf);
-
-    if (n > (uint64_t) std::numeric_limits<T>::max())
-        throw SerialisationError("serialised integer %d is too large for type '%s'", n, typeid(T).name());
-
-    return (T) n;
-}
-
+T readNum(Source & source);
 
 inline unsigned int readInt(Source & source)
 {