nix: improve substituter speed

overlays/default.nix

@@ -69,6 +69,19 @@ with final; {
 
   mlat-client = python3Packages.callPackage ./mlat-client.nix { };
 
+  nixVersions = prev.nixVersions // {
+    stable = prev.nixVersions.stable.overrideAttrs (oldAttrs: {
+      patches = oldAttrs.patches or [ ] ++ [
+        ./nix-increase-substituter-threadpool.diff
+        # request compression
+        (fetchpatch {
+          url = "https://github.com/NixOS/nix/pull/7712.patch";
+          sha256 = "sha256-84C27RxOYi/pamvWLJ/NWsc9zf6VhUAA57llIn8DAT4=";
+        })
+      ];
+    });
+  };
+
   openssh = prev.openssh.overrideAttrs (_: {
     # takes 30 minutes
     doCheck = false;

overlays/nix-increase-substituter-threadpool.diff

@@ -0,0 +1,14 @@
+diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
+index fb985c97b..a509d6f4d 100644
+--- a/src/libstore/misc.cc
++++ b/src/libstore/misc.cc
+@@ -102,7 +102,8 @@ void Store::queryMissing(const std::vector<DerivedPath> & targets,
+     downloadSize_ = narSize_ = 0;
+ 
+     // FIXME: make async.
+-    ThreadPool pool(fileTransferSettings.httpConnections);
++	// https://github.com/NixOS/nix/issues/5118
++    ThreadPool pool(300);
+ 
+     struct State
+     {