Some agenda commands slow down when the Org files grow in size or number. Here are tips to speed up:
(setq org-agenda-dim-blocked-tasks nil)
(setq org-agenda-inhibit-startup nil)
(setq org-agenda-use-tag-inheritance nil)
These options can be applied to selected agenda views. For more details about generation of agenda views, see the docstrings for the relevant variables, and this dedicated Worg page for agenda optimization.