Measurement one: queries on a page with no calendar

Install Query Monitor. Open your front page — assuming there is no calendar on it — and note the total query count. Deactivate the calendar plugin, reload, note it again. The difference is what the calendar costs you on every page of your site that has nothing to do with events.

A well-behaved calendar registers its post type and its block, and then does nothing until something asks it to draw a calendar. The correct difference is zero queries.

Measurement two: autoloaded option size

In Query Monitor, look for the autoloaded options total. WordPress reads every one of these on every request, including admin-ajax calls and REST requests. A healthy site is under a few hundred kilobytes in total; plugins that cache into options without an expiry can add megabytes to that number on their own.

A calendar's fair share is a settings array and possibly a counter. If activating one adds more than a few kilobytes to the autoloaded total, look at what it is storing there.

Measurement three: the posts table

Run SELECT post_type, COUNT(*) FROM wp_posts GROUP BY post_type in phpMyAdmin or your host's database tool. If your calendar's post type has hundreds of rows and you only created a dozen events, the plugin is writing one row per occurrence of your repeating events. That is the single biggest structural cost in this category, and it compounds every week you leave it running.

Measurement four: scheduled tasks

Query Monitor lists scheduled events. WordPress cron does not run on a timer — it runs when somebody visits your site. So a plugin that schedules heavy work is not doing it quietly in the background at 3am; it is doing it in front of whichever visitor happens to trigger it, and they wait for it.

A calendar should have very few of these, and none that expand recurring events. This plugin's free version schedules nothing at all; the paid version schedules two things — fetching your feeds and checking your licence — and both fail quietly and never block a page a visitor is looking at.

The pattern behind all four

Every one of these costs comes from the same decision: doing work ahead of time and storing the result, rather than doing it when it is asked for. Ahead-of-time feels faster and is the reason so many calendars are heavy. Expanding a recurrence rule when the page is drawn takes microseconds; storing its results costs you forever.