I have a client whose site is crashing every time they send an email blast to their database of 20,000 via Mailchimp. What's actually happening is, it will spin for about 5 minutes before first contenful paint.
They used to send to the whole database is one go, but now they schedule batches of 5 every 10 minutes, so effectively it's only sending 4,000 emails each time, and the problem is the same.
It's a CS-Cart 4.4.2 site with 3 storefronts.
Site is hosted with VentraIP on a VPS with 4 vCPU cores and 4GB memory.
When I check the server's resources in WHM, it doesn't even come close to using what's allocated, even during these email campaigns, but the site buckles almost immediately once the email is sent.
What's frustrating is that I can't see the actual concurrent users in Google Analytics, presumably because they're not loading enough of the site to trigger the script, and the Mailchimp report usually only shows a few clicks at the time it's happening (they told me the clicks showing on the report is basically in real time). But it's hard to believe that more than 30 users are INSTANTLY on the site at the same time.
The site is caching on AWS and we've already upgraded the VPS recently, and the client is starting to get frustrated that there's no clear answer.
What could be the possible source/s of this issue, and how can I confirm it?
Thanks,
Matt