diff --git a/t/helper/test-genzeros.c b/t/helper/test-genzeros.c
index b1197e91a8..8ca988d621 100644
--- a/t/helper/test-genzeros.c
+++ b/t/helper/test-genzeros.c
@@ -3,7 +3,10 @@
 
 int cmd__genzeros(int argc, const char **argv)
 {
+	/* static, so that it is NUL-initialized */
+	static const char zeros[256 * 1024];
 	intmax_t count;
+	ssize_t n;
 
 	if (argc > 2) {
 		fprintf(stderr, "usage: %s [<count>]\n", argv[0]);
@@ -12,9 +15,19 @@ int cmd__genzeros(int argc, const char **argv)
 
 	count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
 
-	while (count < 0 || count--) {
-		if (putchar(0) == EOF)
+	/* Writing out individual NUL bytes is slow... */
+	while (count < 0)
+		if (write(1, zeros, ARRAY_SIZE(zeros)) < 0)
 			return -1;
+
+	while (count > 0) {
+		n = write(1, zeros, count < ARRAY_SIZE(zeros) ?
+			  count : ARRAY_SIZE(zeros));
+
+		if (n < 0)
+			return -1;
+
+		count -= n;
 	}
 
 	return 0;