diff --git a/src/libfetchers/fetch-to-store.cc b/src/libfetchers/fetch-to-store.cc
index 65aa72a6c36..d9fff8cd45c 100644
--- a/src/libfetchers/fetch-to-store.cc
+++ b/src/libfetchers/fetch-to-store.cc
@@ -1,6 +1,7 @@
 #include "fetch-to-store.hh"
 #include "fetchers.hh"
 #include "cache.hh"
+#include "posix-source-accessor.hh"
 
 namespace nix {
 
@@ -13,8 +14,16 @@ StorePath fetchToStore(
     PathFilter * filter,
     RepairFlag repair)
 {
-    // FIXME: add an optimisation for the case where the accessor is
-    // a `PosixSourceAccessor` pointing to a store path.
+    if (path.accessor->isStorePath
+        && path.path.isRoot()
+        && method == FileIngestionMethod::Recursive
+        && !filter)
+    {
+        if (auto accessor = path.accessor.dynamic_pointer_cast<PosixSourceAccessor>())
+            if (auto storePath = store.maybeParseStorePath(accessor->root.string()))
+                if (storePath->name() == name)
+                    return *storePath;
+    }
 
     std::optional<fetchers::Cache::Key> cacheKey;
 
diff --git a/src/libfetchers/path.cc b/src/libfetchers/path.cc
index 68958d55971..8ba873b3722 100644
--- a/src/libfetchers/path.cc
+++ b/src/libfetchers/path.cc
@@ -155,7 +155,12 @@ struct PathInputScheme : InputScheme
         }
         input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
 
-        return {makeStorePathAccessor(store, *storePath), std::move(input)};
+        auto accessor = makeStorePathAccessor(store, *storePath);
+
+        /* Optimize fetchToStore() calls on this path. */
+        accessor->isStorePath = true;
+
+        return {accessor, std::move(input)};
     }
 
     std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
diff --git a/src/libfetchers/unix/mercurial.cc b/src/libfetchers/unix/mercurial.cc
index 7bdf1e9375b..e2fba21ca8a 100644
--- a/src/libfetchers/unix/mercurial.cc
+++ b/src/libfetchers/unix/mercurial.cc
@@ -339,6 +339,9 @@ struct MercurialInputScheme : InputScheme
 
         accessor->setPathDisplay("«" + input.to_string() + "»");
 
+        /* Optimize fetchToStore() calls on this path. */
+        accessor->isStorePath = true;
+
         return {accessor, input};
     }
 
diff --git a/src/libutil/source-accessor.hh b/src/libutil/source-accessor.hh
index d7fb0af5fac..75c9ec8faed 100644
--- a/src/libutil/source-accessor.hh
+++ b/src/libutil/source-accessor.hh
@@ -181,6 +181,13 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
      */
     virtual std::optional<time_t> getLastModified()
     { return std::nullopt; }
+
+    /**
+     * Whether this is a store path using
+     * FileIngestionMethod::Recursive. This is used to optimize
+     * `fetchToStore()`.
+     */
+    bool isStorePath = false;
 };
 
 /**