Basically happens with all notifications. Most notably from Whatsapp web. Opera freezes and becomes unresponsive for like 5 mins to 10 mins.
OS: Win 10 Pro (64bit)
Browser Identification: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36 OPR/44.0.2510.857