How to understand why a site without visitors on WP loads the server to failure?

Question

Network administration

I don’t know where to look already ... They
gave me the finished site, I try to place it on VPS 512Mb RAM, 16Gb SSD - the hoster deployed CentOS 7 there of my choice and I installed Vesta CP (downgrading PHP after to 7.1 (because there is why - then it’s already set to 7.4RC) I host the
site and the server can think straight, so it stops responding after trying to enter the admin panel or the site itself.(sometimes it does, but everything moves very slowly). /yadi.sk/i/nvlVxvLG1dYBUA - in atop at that time such a picture (this is when he still answers) SWP to 1Gb I already increased it, there was 30Mb and it was impossible to open the site at all - almost immediately OMKiller killed mysql

In general - what to do? and where to look. the site is simple, there are at most 20 pages, nothing tricky, there is no traffic to the site yet ...
There is, of course, the thought that this might be wrong with the server, but here my knowledge is not enough to understand this. I measured the speed of writing and reading a file according to instructions on the Internet - it showed little, technical support replied that the following limits were on the tariff:
Read IOPS ≈ 400
Write IOPS ≈ 200
Read bandwidth ≈ 40 MB / s
Write bandwidth ≈ 20 MB / s

Answer 1

• Weak server
  
• Obsolete VestaCP
  
• Load viruses or robots
  
• Not optimally compiled application (Wordpress)
  

Answer 2

read the logs, the answer to everything is there

Answer 3

on local, make Virtuabox with the same 512 / 1 and measure iops and how it will work in general