This is the official manual for the latest Org-mode release.
When your Org files grow in both number and size, agenda commands may start to become slow. Below are some tips on how to speed up the agenda commands.
(setq org-agenda-dim-blocked-tasks nil)
(setq org-agenda-inhibit-startup nil)
(setq org-agenda-use-tag-inheritance nil)
You can set these options for specific agenda views only. See the docstrings of these variables for details on why they affect the agenda generation, and this dedicated Worg page for further explanations.