Diff of my attempt to introduce a status.enumerateUntrackedDelayMS config to Git

git-diff.txt

diff --git a/Documentation/config/status.txt b/Documentation/config/status.txt
index 0fc704ab80..393670fac2 100644
--- a/Documentation/config/status.txt
+++ b/Documentation/config/status.txt
@@ -60,6 +60,13 @@ If this variable is not specified, it defaults to 'normal'.
 This variable can be overridden with the -u|--untracked-files option
 of linkgit:git-status[1] and linkgit:git-commit[1].
 
+status.enumerateUntrackedDelayMS::
+	If the enumeration of untracked files takes longer than a certain
+	amount of time, an advice message will display to guide the user
+	towards the help section that can help them tune their
+	linkgit:git-status[1] performance. That amount of time is 2000ms
+	by default, but this config allows to set it to a different delay.
+
 status.submoduleSummary::
 	Defaults to false.
 	If this is set to a non zero number or true (identical to -1 or an
diff --git a/Documentation/git-status.txt b/Documentation/git-status.txt
index 5e438a7fdc..a051b1e8f3 100644
--- a/Documentation/git-status.txt
+++ b/Documentation/git-status.txt
@@ -457,6 +457,66 @@ during the write may conflict with other simultaneous processes, causing
 them to fail. Scripts running `status` in the background should consider
 using `git --no-optional-locks status` (see linkgit:git[1] for details).
 
+UNTRACKED FILES AND PERFORMANCE
+-------------------------------
+
+`git status` can be very slow in large worktrees if/when it
+needs to search for untracked files and directories. There are
+many configuration options available to speed this up by either
+avoiding the work or making use of cached results from previous
+Git commands. There is no single optimum set of settings right
+for everyone. We'll list a summary of the relevant options to help
+you, but before going into the list, you may want to run `git status`
+again, because your configuration may already be caching `git status`
+results, so it could be faster on subsequent runs.
+
+* The `--untracked-files=no` flag or the
+	`status.showUntrackedfiles=false` config (see above for both):
+	indicate that `git status` should not report untracked
+	files. This is the fastest option. `git status` will not list
+	the untracked files, so you need to be careful to remember if
+	you create any new files and manually `git add` them.
+
+* `advice.statusUoption=false` (see linkgit:git-config[1]):
+	setting this variable to `false` disables the warning message
+	given when enumerating untracked files takes more than 2
+	seconds.  In a large project, it may take longer and the user
+	may have already accepted the trade off (e.g. using "-uno" may
+	not be an acceptable option for the user), in which case, there
+	is no point issuing the warning message, and in such a case,
+	disabling the warning may be the best.
+
+* `core.untrackedCache=true` (see linkgit:git-update-index[1]):
+	enable the untracked cache feature and only search directories
+	that have been modified since the previous `git status` command.
+	Git remembers the set of untracked files within each directory
+	and assumes that if a directory has not been modified, then
+	the set of untracked files within has not changed.  This is much
+	faster than enumerating the contents of every directory, but still
+	not without cost, because Git still has to search for the set of
+	modified directories. The untracked cache is stored in the
+	`.git/index` file. The reduced cost of searching for untracked
+	files is offset slightly by the increased size of the index and
+	the cost of keeping it up-to-date. That reduced search time is
+	usually worth the additional size.
+
+* `core.untrackedCache=true` and `core.fsmonitor=true` or
+	`core.fsmonitor=<hook_command_pathname>` (see
+	linkgit:git-update-index[1]): enable both the untracked cache
+	and FSMonitor features and only search directories that have
+	been modified since the previous `git status` command.  This
+	is faster than using just the untracked cache alone because
+	Git can also avoid searching for modified directories.  Git
+	only has to enumerate the exact set of directories that have
+	changed recently. While the FSMonitor feature can be enabled
+	without the untracked cache, the benefits are greatly reduced
+	in that case.
+
+Note that after you turn on the untracked cache and/or FSMonitor
+features it may take a few `git status` commands for the various
+caches to warm up before you see improved command times.  This is
+normal.
+
 SEE ALSO
 --------
 linkgit:gitignore[5]
diff --git a/builtin/commit.c b/builtin/commit.c
index e22bdf23f5..ee614b8f52 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1457,6 +1457,10 @@ static int git_status_config(const char *k, const char *v, void *cb)
 			return error(_("Invalid untracked files mode '%s'"), v);
 		return 0;
 	}
+	if (!strcmp(k, "status.enumerateuntrackeddelayms")) {
+		s->enumerate_untracked_delay_ms = git_config_int(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "diff.renamelimit")) {
 		if (s->rename_limit == -1)
 			s->rename_limit = git_config_int(k, v);
diff --git a/t/t7508-status.sh b/t/t7508-status.sh
index 2b7ef6c41a..0333dc40a8 100755
--- a/t/t7508-status.sh
+++ b/t/t7508-status.sh
@@ -1676,4 +1676,80 @@ test_expect_success 'racy timestamps will be fixed for dirty worktree' '
 	! test_is_magic_mtime .git/index
 '
 
+test_expect_success 'setup slow status advice' '
+	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main git init slowstatus &&
+	(
+		cd slowstatus &&
+		cat >.gitignore <<-\EOF &&
+		/actual
+		/expected
+		/out
+		EOF
+		git add .gitignore &&
+		git commit -m "Add .gitignore" &&
+		git config advice.statusuoption true
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache and fsmonitor are unset' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache false &&
+		git config core.fsmonitor false &&
+		git config core.enumerateuntrackeddelayms -1 &&
+		git status >out &&
+		sed "s/ [0-9][0-9]*\.[0-9][0-9]/ X/" out >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took X seconds to enumerate untracked files.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache true, but not fsmonitor' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache true &&
+		git config core.fsmonitor false &&
+		git config core.enumerateuntrackeddelayms -1 &&
+		git status >out &&
+		sed "s/ [0-9][0-9]*\.[0-9][0-9]/ X/" out >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took X seconds to enumerate untracked files.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
+test_expect_success 'slow status advice when core.untrackedCache true, and fsmonitor' '
+	(
+		cd slowstatus &&
+		git config core.untrackedCache true &&
+		git config core.fsmonitor true &&
+		git config core.enumerateuntrackeddelayms -1 &&
+		git status >out &&
+		sed "s/ [0-9][0-9]*\.[0-9][0-9]/ X/" out >actual &&
+		cat >expected <<-\EOF &&
+		On branch main
+
+		It took X seconds to enumerate untracked files,
+		but the results were cached, and subsequent runs may be faster.
+		See '\''git help status'\'' for information on how to improve this.
+
+		nothing to commit, working tree clean
+		EOF
+		test_cmp expected actual
+	)
+'
+
 test_done
diff --git a/wt-status.c b/wt-status.c
index 5813174896..80107c4f9e 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -18,6 +18,7 @@
 #include "worktree.h"
 #include "lockfile.h"
 #include "sequencer.h"
+#include "fsmonitor-settings.h"
 
 #define AB_DELAY_WARNING_IN_MS (2 * 1000)
 
@@ -142,6 +143,7 @@ void wt_status_prepare(struct repository *r, struct wt_status *s)
 	s->show_stash = 0;
 	s->ahead_behind_flags = AHEAD_BEHIND_UNSPECIFIED;
 	s->display_comment_prefix = 0;
+	s->enumerate_untracked_delay_ms = 2000;
 	s->detect_rename = -1;
 	s->rename_score = -1;
 	s->rename_limit = -1;
@@ -1205,6 +1207,11 @@ static void wt_longstatus_print_tracking(struct wt_status *s)
 	strbuf_release(&sb);
 }
 
+static int uf_was_slow(struct wt_status *s)
+{
+	return s->enumerate_untracked_delay_ms <= s->untracked_in_ms;
+}
+
 static void show_merge_in_progress(struct wt_status *s,
 				   const char *color)
 {
@@ -1814,6 +1821,7 @@ static void wt_longstatus_print(struct wt_status *s)
 {
 	const char *branch_color = color(WT_STATUS_ONBRANCH, s);
 	const char *branch_status_color = color(WT_STATUS_HEADER, s);
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(s->repo);
 
 	if (s->branch) {
 		const char *on_what = _("On branch ");
@@ -1870,13 +1878,21 @@ static void wt_longstatus_print(struct wt_status *s)
 		wt_longstatus_print_other(s, &s->untracked, _("Untracked files"), "add");
 		if (s->show_ignored_mode)
 			wt_longstatus_print_other(s, &s->ignored, _("Ignored files"), "add -f");
-		if (advice_enabled(ADVICE_STATUS_U_OPTION) && 2000 < s->untracked_in_ms) {
+		if (advice_enabled(ADVICE_STATUS_U_OPTION) && uf_was_slow(s)) {
 			status_printf_ln(s, GIT_COLOR_NORMAL, "%s", "");
+			if (fsm_mode > FSMONITOR_MODE_DISABLED) {
+				status_printf_ln(s, GIT_COLOR_NORMAL,
+						_("It took %.2f seconds to enumerate untracked files,\n"
+						"but the results were cached, and subsequent runs may be faster."),
+						s->untracked_in_ms / 1000.0);
+			} else {
+				status_printf_ln(s, GIT_COLOR_NORMAL,
+						_("It took %.2f seconds to enumerate untracked files."),
+						s->untracked_in_ms / 1000.0);
+			}
 			status_printf_ln(s, GIT_COLOR_NORMAL,
-					 _("It took %.2f seconds to enumerate untracked files. 'status -uno'\n"
-					   "may speed it up, but you have to be careful not to forget to add\n"
-					   "new files yourself (see 'git help status')."),
-					 s->untracked_in_ms / 1000.0);
+					_("See 'git help status' for information on how to improve this."));
+			status_printf_ln(s, GIT_COLOR_NORMAL, "%s", "");
 		}
 	} else if (s->committable)
 		status_printf_ln(s, GIT_COLOR_NORMAL, _("Untracked files not listed%s"),
diff --git a/wt-status.h b/wt-status.h
index ab9cc9d8f0..bccf3da94e 100644
--- a/wt-status.h
+++ b/wt-status.h
@@ -112,6 +112,7 @@ struct wt_status {
 	int use_color;
 	int no_gettext;
 	int display_comment_prefix;
+	int enumerate_untracked_delay_ms;
 	int relative_paths;
 	int submodule_summary;
 	enum show_ignored_type show_ignored_mode;