diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index bca5674e3545ae..9cd997cce61cfe 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -927,7 +927,7 @@ pub fn (s string) split_nth(delim string, nth int) []string {
 		0 {
 			for i, ch in s {
 				if nth > 0 && res.len == nth - 1 {
-					res << s[i..]
+					res << unsafe { s.substr_unsafe(i, s.len - 1) }
 					break
 				}
 				res << ch.ascii_str()
@@ -941,7 +941,7 @@ pub fn (s string) split_nth(delim string, nth int) []string {
 					if nth > 0 && res.len == nth - 1 {
 						break
 					}
-					res << s.substr(start, i)
+					res << unsafe { s.substr_unsafe(start, i) }
 					start = i + 1
 				}
 			}
@@ -953,18 +953,18 @@ pub fn (s string) split_nth(delim string, nth int) []string {
 			mut start := 0
 			// Add up to `nth` segments left of every occurrence of the delimiter.
 			for i := 0; i <= s.len; i++ {
-				if s[i..i + delim.len] or { break } == delim {
+				if i + delim.len <= s.len && unsafe { s.substr_unsafe(i, i + delim.len) } == delim {
 					if nth > 0 && res.len == nth - 1 {
 						break
 					}
-					res << s.substr(start, i)
+					res << unsafe { s.substr_unsafe(start, i) }
 					i += delim.len
 					start = i
 				}
 			}
 			// Then add the remaining part of the string as the last segment.
 			if nth < 1 || res.len < nth {
-				res << s[start..]
+				res << unsafe { s.substr_unsafe(start, s.len - 1) }
 			}
 		}
 	}
@@ -984,7 +984,7 @@ pub fn (s string) rsplit_nth(delim string, nth int) []string {
 		0 {
 			for i := s.len - 1; i >= 0; i-- {
 				if nth > 0 && res.len == nth - 1 {
-					res << s[..i + 1]
+					res << unsafe { s.substr_unsafe(0, i + 1) }
 					break
 				}
 				res << s[i].ascii_str()
@@ -998,29 +998,30 @@ pub fn (s string) rsplit_nth(delim string, nth int) []string {
 					if nth > 0 && res.len == nth - 1 {
 						break
 					}
-					res << s[i + 1..rbound]
+					res << unsafe { s.substr_unsafe(i + 1, rbound) }
 					rbound = i
 				}
 			}
 			if nth < 1 || res.len < nth {
-				res << s[..rbound]
+				res << unsafe { s.substr_unsafe(0, rbound) }
 			}
 		}
 		else {
 			mut rbound := s.len
 			for i := s.len - 1; i >= 0; i-- {
-				is_delim := i - delim.len >= 0 && s[i - delim.len..i] == delim
+				is_delim := i - delim.len >= 0
+					&& unsafe { s.substr_unsafe(i - delim.len, i) } == delim
 				if is_delim {
 					if nth > 0 && res.len == nth - 1 {
 						break
 					}
-					res << s[i..rbound]
+					res << unsafe { s.substr_unsafe(i, rbound) }
 					i -= delim.len
 					rbound = i
 				}
 			}
 			if nth < 1 || res.len < nth {
-				res << s[..rbound]
+				res << unsafe { s.substr_unsafe(0, rbound) }
 			}
 		}
 	}