Improve WordPress performance on large number of posts

You could activate the Query Monitor plugin and then load the post(s) / page(s) that are giving you trouble. It will show you where the slowness is being caused, core, theme, plugin A or Plugin B.

It is often possible to speed up a slow query by adding the appropriate index. Try reading this article for tips on how to do this.

Anything more specific will require much more information from you on exactly what is wrong. Details, details.