diff --git a/string.c b/string.c
index 711918b..d4cbcbe 100644
--- a/string.c
+++ b/string.c
@@ -406,10 +406,19 @@ str_new(VALUE klass, const char *ptr, long len)
     return str;
 }
 
+extern void rb_obj_call_init_fast(VALUE obj, int argc, VALUE *argv);
+
 VALUE
 rb_str_new(const char *ptr, long len)
 {
-    return str_new(rb_cString, ptr, len);
+    /* str is already initialized by original initialize within str_new() */
+    VALUE str = str_new(rb_cString, ptr, len);
+
+    /* speed: call initialize only if it was redefined. See #4893 */
+    /* passing str itself as parameter, avoiding need for a temp str */
+    rb_obj_call_init_fast((VALUE)str, 1, &str);
+
+    return str;
 }
 
 VALUE
diff --git a/vm_method.c b/vm_method.c
index 86ff2ee..43c50b9 100644
--- a/vm_method.c
+++ b/vm_method.c
@@ -258,6 +258,34 @@ method_added(VALUE klass, ID mid)
     }
 }
 
+/* use class-variable (bit-flag). TODO: move to header file */
+#define FL_CALL_INIT FL_USER18
+
+void
+rb_set_call_flags(VALUE klass, ID mid, int set)
+{
+/*  if used for more classes, introduce FL_FAST_CLASS to mark*/
+
+	if (ruby_running && mid == idInitialize && klass == rb_cString) {
+		if (set) {
+			FL_SET(klass, FL_CALL_INIT);	    
+		} else {
+			FL_UNSET(klass, FL_CALL_INIT);	    
+		}
+	}
+}
+
+void
+rb_obj_call_init_fast(VALUE obj, int argc, VALUE *argv)
+{
+/*  use only in fast_classes like String. Method lookup and call of initialize
+    will be done only, if initialize was redefined. */
+
+	if (FL_TEST(CLASS_OF(obj), FL_CALL_INIT)) {
+		rb_obj_call_init(obj, argc, argv);
+	}	
+}
+
 rb_method_entry_t *
 rb_add_method(VALUE klass, ID mid, rb_method_type_t type, void *opts, rb_method_flag_t noex)
 {
@@ -304,6 +332,9 @@ rb_add_method(VALUE klass, ID mid, rb_method_type_t type, void *opts, rb_method_
       default:
 	rb_bug("rb_add_method: unsupported method type (%d)\n", type);
     }
+
+    rb_set_call_flags(klass, mid, TRUE);
+
     if (type != VM_METHOD_TYPE_UNDEF) {
 	method_added(klass, mid);
     }
@@ -441,7 +472,7 @@ remove_method(VALUE klass, ID mid)
     rb_vm_check_redefinition_opt_method(me);
     rb_clear_cache_for_undef(klass, mid);
     rb_unlink_method_entry(me);
-
+    rb_set_call_flags(klass, mid, FALSE);
     CALL_METHOD_HOOK(klass, removed, mid);
 }
 
@@ -628,6 +659,8 @@ rb_undef(VALUE klass, ID id)
 
     rb_add_method(klass, id, VM_METHOD_TYPE_UNDEF, 0, NOEX_PUBLIC);
 
+    rb_set_call_flags(klass, id, FALSE);
+
     CALL_METHOD_HOOK(klass, undefined, id);
 }