This is because the string concatenation operations can produce too many intermediate strings, making it perform poorly.
As the number of concatenation operations increases, it can be better to use another concatenation method like the Array.join() function, which joins the strings together into an array.
An alternative method is to use the string.concat() method to join the strings together. This example demonstrates the use of this method:
Modern browsers support all three of these methods, but the browser performance can differ between browsers and even versions of the same browser. To optimize your code, write a sample of code that uses each method a lot of times (one million, for example), time the results with different browsers, and see which is consistently the fastest.
Use the fastest for your string operations for optimum results.