diff --git a/vm/src/anystr.rs b/vm/src/anystr.rs
index 7cd88926e..ba7cd17d2 100644
--- a/vm/src/anystr.rs
+++ b/vm/src/anystr.rs
@@ -145,7 +145,9 @@ pub trait AnyStr<'s>: 's {
     // FIXME: get_chars is expensive for str
     fn get_chars(&self, range: std::ops::Range<usize>) -> &Self;
     fn bytes_len(&self) -> usize;
-    fn chars_len(&self) -> usize; // cannot access to cache here
+    // NOTE: str::chars().count() consumes the O(n) time. But pystr::char_len does cache.
+    //       So using chars_len directly is too expensive and the below method shouldn't be implemented.
+    // fn chars_len(&self) -> usize;
     fn is_empty(&self) -> bool;
 
     fn py_add(&self, other: &Self) -> Self::Container {
diff --git a/vm/src/builtins/pystr.rs b/vm/src/builtins/pystr.rs
index 64c46d20e..ec3ea2ae7 100644
--- a/vm/src/builtins/pystr.rs
+++ b/vm/src/builtins/pystr.rs
@@ -1645,10 +1645,6 @@ impl<'s> AnyStr<'s> for str {
         Self::len(self)
     }
 
-    fn chars_len(&self) -> usize {
-        self.chars().count()
-    }
-
     fn py_split_whitespace<F>(&self, maxsplit: isize, convert: F) -> Vec<PyObjectRef>
     where
         F: Fn(&Self) -> PyObjectRef,
diff --git a/vm/src/bytesinner.rs b/vm/src/bytesinner.rs
index 8becd5592..61c391710 100644
--- a/vm/src/bytesinner.rs
+++ b/vm/src/bytesinner.rs
@@ -1073,10 +1073,6 @@ impl<'s> AnyStr<'s> for [u8] {
         Self::len(self)
     }
 
-    fn chars_len(&self) -> usize {
-        Self::len(self)
-    }
-
     fn py_split_whitespace<F>(&self, maxsplit: isize, convert: F) -> Vec<PyObjectRef>
     where
         F: Fn(&Self) -> PyObjectRef,