There are so many performance improved in 11.x version. Before this version if a user open multiple tab this produce multiple requests but in this version no matter how many tabs are open in same browser it will just make 1 request per interval.
Also previous versions it used admin ajax but now use wp-json which is also a performance improved. It use less resources.
After adding that code wait couple of hours. Next time user refresh their browser will load that code and reduce call.
I suggest you use 11.x version. It is better in performance. After changing that code just wait couple of hours to load in users browser. You can change to 10 minutes more if required. This notification call is present in 10.x version also. But it use admin ajax thats why you may not find it. But for ajax call wp-json in better for performance