diff --git a/src/basic/string-util.c b/src/basic/string-util.c
index 9d2f4bc8f9..c3d97740d2 100644
--- a/src/basic/string-util.c
+++ b/src/basic/string-util.c
@@ -635,6 +635,11 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz) {
         if (!f)
                 return NULL;
 
+        /* Note we use the _unlocked() stdio variants on f for performance
+         * reasons.  It's safe to do so since we created f here and it
+         * doesn't leave our scope.
+         */
+
         for (i = *ibuf; i < *ibuf + isz + 1; i++) {
 
                 switch (state) {
@@ -645,21 +650,21 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz) {
                         else if (*i == '\x1B')
                                 state = STATE_ESCAPE;
                         else if (*i == '\t')
-                                fputs("        ", f);
+                                fputs_unlocked("        ", f);
                         else
-                                fputc(*i, f);
+                                fputc_unlocked(*i, f);
                         break;
 
                 case STATE_ESCAPE:
                         if (i >= *ibuf + isz) { /* EOT */
-                                fputc('\x1B', f);
+                                fputc_unlocked('\x1B', f);
                                 break;
                         } else if (*i == '[') {
                                 state = STATE_BRACKET;
                                 begin = i + 1;
                         } else {
-                                fputc('\x1B', f);
-                                fputc(*i, f);
+                                fputc_unlocked('\x1B', f);
+                                fputc_unlocked(*i, f);
                                 state = STATE_OTHER;
                         }
 
@@ -669,8 +674,8 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz) {
 
                         if (i >= *ibuf + isz || /* EOT */
                             (!(*i >= '0' && *i <= '9') && *i != ';' && *i != 'm')) {
-                                fputc('\x1B', f);
-                                fputc('[', f);
+                                fputc_unlocked('\x1B', f);
+                                fputc_unlocked('[', f);
                                 state = STATE_OTHER;
                                 i = begin-1;
                         } else if (*i == 'm')