Slow down server/website when new notification is send

Hi, whenever I publish new post and send new notification; my website starts to slow down (very slow loading) and finally become unreachable. Website remain unreachable for 5 to 8 minutes and then gradually come back to normal. If I login to my server (WHM) and restart Apache or mySql then website start functioning as normal as soon as I restart.

I tried many things to fix the issue but fails. For e.g. I have tried to improve mysql performance by doing ___ skip-name-resolve and tried every recommendation which script mysqltuner.pl suggest.
When server goes down, I do not see any alarming warning in the log file

I have a good server so I don’t think my server will be issue. I am using Dedicated Cloud server from vultr.com, it has 16 GB RAM and 6 CPU and dedicated IP. Website’s SSL is from Comodo.

My website have almost 3 million page-views per month.
PHP Version is 7.0 and using PHP FPM and HTTP 2
I am not using any cache plugin or pagespeed module as well as not using any Security plugin such as wp security/wordfence or JetPack

I have 20000+ plus onesignal subscriber. And my logo/icon which is shown in notification is hosted in Imgur.com

I direly need onesignal but due to the above issue I am not sending any new notification. Please suggest what could be the issue and where to look for.