Do Widgets Slow Notion Down?
A little, and it depends almost entirely on how many. One or two widgets and you will never notice. Around five or six is where a dashboard starts to feel slow to open, and on a phone with patchy signal you notice sooner. Weight matters as much as count, because one heavy widget can cost more than four light ones.
Why count matters
Every embed is a whole web page loading inside your page. Six embeds means six separate things fetching before the page settles, and they do not all finish at the same speed.
Why weight matters more
A divider is a few kilobytes and appears instantly. Anything animated, or anything pulling live data from elsewhere, does considerably more work. Counting blocks only tells you half the story.
What to do about it
None of this is a reason to avoid widgets, just a reason to be deliberate. If a page got noticeably slower after you built it, take one embed off and see whether the speed comes back.
The same applies to long pages generally, widgets or not. A Notion page with a thousand blocks takes a moment regardless of what is in it, and it is harder to read besides. Sometimes the fix is two pages, not fewer widgets.
Which ones tend to be worth their weight is in best Notion widgets.
Other platforms, same arithmetic
Coda, Craft and xTiles behave much the same, since the cost sits in the embedded page rather than the host. WordPress can be better or worse depending on the theme: lazy loading helps, a heavy theme on top of heavy embeds does not.
Wherever you are, the habit is identical. Count the embeds on a page before adding another, and if it is already at five, ask whether the new one replaces something rather than joining it.