diff --git a/builtin/stash.c b/builtin/stash.c
index 32dbc97b47..c4809f299a 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -372,6 +372,56 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	return 0;
 }
 
+static int create_index_from_tree(const struct object_id *tree_id,
+				  const char *index_path)
+{
+	int nr_trees = 1;
+	int ret = 0;
+	struct unpack_trees_options opts;
+	struct tree_desc t[MAX_UNPACK_TREES];
+	struct tree *tree;
+	struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
+	struct lock_file lock_file = LOCK_INIT;
+
+	repo_read_index_preload(the_repository, NULL, 0);
+	refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
+
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+
+	memset(&opts, 0, sizeof(opts));
+
+	tree = repo_parse_tree_indirect(the_repository, tree_id);
+	if (!tree || repo_parse_tree(the_repository, tree)) {
+		ret = -1;
+		goto done;
+	}
+
+	init_tree_desc(t, &tree->object.oid, tree->buffer, tree->size);
+
+	opts.head_idx = 1;
+	opts.src_index = the_repository->index;
+	opts.dst_index = &dst_istate;
+	opts.merge = 1;
+	opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+	opts.fn = oneway_merge;
+
+	if (unpack_trees(nr_trees, t, &opts)) {
+		ret = -1;
+		goto done;
+	}
+
+	if (write_locked_index(&dst_istate, &lock_file, COMMIT_LOCK)) {
+		ret = error(_("unable to write new index file"));
+		goto done;
+	}
+
+done:
+	release_index(&dst_istate);
+	if (ret)
+		rollback_lock_file(&lock_file);
+	return ret;
+}
+
 static int diff_tree_binary(struct strbuf *out, struct object_id *w_commit)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -1321,18 +1371,26 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 		       struct interactive_options *interactive_opts)
 {
 	int ret = 0;
-	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
+	struct commit *head_commit;
+	const struct object_id *head_tree;
 	struct index_state istate = INDEX_STATE_INIT(the_repository);
 	char *old_index_env = NULL, *old_repo_index_file;
 
 	remove_path(stash_index_path.buf);
 
-	cp_read_tree.git_cmd = 1;
-	strvec_pushl(&cp_read_tree.args, "read-tree", "HEAD", NULL);
-	strvec_pushf(&cp_read_tree.env, "GIT_INDEX_FILE=%s",
-		     stash_index_path.buf);
-	if (run_command(&cp_read_tree)) {
+	head_commit = lookup_commit(the_repository, &info->b_commit);
+	if (!head_commit || repo_parse_commit(the_repository, head_commit)) {
+		ret = -1;
+		goto done;
+	}
+	head_tree = get_commit_tree_oid(head_commit);
+	if (!head_tree) {
+		ret = -1;
+		goto done;
+	}
+
+	if (create_index_from_tree(head_tree, stash_index_path.buf)) {
 		ret = -1;
 		goto done;
 	}
diff --git a/t/perf/p3904-stash-patch.sh b/t/perf/p3904-stash-patch.sh
new file mode 100755
index 0000000000..4cfce638be
--- /dev/null
+++ b/t/perf/p3904-stash-patch.sh
@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description="Performance tests for git stash -p"
+
+. ./perf-lib.sh
+
+test_perf_fresh_repo
+
+test_expect_success "setup" '
+	mkdir files &&
+	test_seq 1 100000 | while read i; do
+		echo "content $i" >files/$i.txt || return 1
+	done &&
+	git add files/ &&
+	git commit -q -m "add tracked files" &&
+	echo modified >files/1.txt
+'
+
+test_perf "stash -p, no fsmonitor" \
+	--setup 'echo modified >files/1.txt' '
+	printf "q\n" | git stash -p >/dev/null 2>&1 || true
+'
+
+if test_have_prereq FSMONITOR_DAEMON
+then
+	test_expect_success "enable builtin fsmonitor" '
+		git config core.fsmonitor true &&
+		git fsmonitor--daemon start &&
+		git update-index --fsmonitor &&
+		git status >/dev/null 2>&1
+	'
+
+	test_perf "stash -p, builtin fsmonitor" \
+		--setup 'echo modified >files/1.txt && git status >/dev/null 2>&1' '
+		printf "q\n" | git stash -p >/dev/null 2>&1 || true
+	'
+
+	test_expect_success "stop builtin fsmonitor" '
+		git fsmonitor--daemon stop
+	'
+fi
+
+test_done