How to reduce wpms memory usage?

Try using a plugin like Query Monitor to get an overview of what’s shown for each page load.

This will allow you to check:

Database queries, including for slow, duplicate, or erroneous queries. Either refactoring or using transient / object caching for repeated database calls.
Resolve an PHP errors in the call stack and corresponding component.
Remove unnecessary enqueued scripts and stylesheets, along with their dependencies, dependents.
Additional ideas may include, using php code sniffer which helps detect violations of pre-defined coding standards, and has a tool to automatically fix them.

A final solutions is to have an app like new relic installed on the server to gian a better understand of the overall application performance, dependencies, and bottlenecks.