Analyze and solve load problems

what we have:
hosting sweb.ru
on account - 11 sites
СMS: WP, MODx, Host CMS, OpenCart

what we need to do:
- to analyze site load on the hosting server (there are statistics and logs on the server, awstats)
- to make an estimate of the cost of upcoming work
- to carry out work to reduce the load

At the moment, by means of redirects, it was revealed that the load is created by 2 site on CMS WORDPRESS.
Moreover, the load increased at the moment of connection to the site of personal subscriptions to news for mailing and the possibility of tracking the order of one of the sites (maybe a coincidence)