Fix critical errors on your Wordpress site

Errors appeared on our woocommerce site, due to which the site does not function normally. Customers periodically fail to place an order, and then the site crashes with a critical error.

You need to find out the cause of the memory devouring and optimize the code.

Detailed TK here: https://docs.google.com/document/d/1f-4UWNIsWErL3c1KHJWc3EnwXYJ9vBgHcuG7f5RCiJE/edit?usp=sharing

Requirements:
- Good command of php
- You know how to refactor code
- You have experience with databases

Please, in response:
1. Enter your experience in PHP
2. Briefly describe how you will solve the problem
3. Orient by cost and timing