fix: faster transparentize by using string index instead of replace

lib/mixins/canvas-drawer.js

@@ -946,12 +946,13 @@ function drawCustomDecoration (decoration, data, decorationColor, editorElement)
  * opacity.
  *
  * @param  {string} color the CSS RGB color to transparentize
- * @param  {number} [opacity=1] the opacity amount
+ * @param  {number} opacity the opacity amount
  * @return {string} the transparentized CSS color
  * @access private
  */
-function transparentize (color, opacity = 1) {
-  return color.replace('rgb(', 'rgba(').replace(')', `, ${opacity})`)
+function transparentize (color, opacity) {
+  // assumes that color is in form of `rgb(content)` with no spaces around the given value
+  return `rgba(${color.slice(4, -1)}, ${opacity})`
 }
 
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