WooCommerce Product Vendor Plugin
This provides a fix that works for a large number of vendors on a shared server. Tested to work with 15k+ vendors on WPEngine start-up plan shared server.
The issue is that woocommerce product vendor plugin queries for all vendors whenever a vendor manager logins and then searches through the entire vendor list to find out which vendors are managed by the given user. This results in long SQL queries that are killed on a shared server.
We have fixed the issue by introducing a user meta field _sc_wcpv_vendor that stores the vendor id for each user.