Everyone’s posts above have now pretty much mentioned all the potential points for slowdown now. If this was your first post, and you also hadn’t heard of regex, I think you’re pretty safe to not be worrying about them yet 
They tend to be things that can be solved when they occur. i.e. They’re not deeply systemic in terms of code volume, except maybe using heavy relations. Pathfinding, regex, these tend to be small bits of code you write, that get used a lot, and if you find they’re slowing you down, those small bits need to be rewritten to be more efficient, or less trigger-happy.
That second point especially. If we look at two of the biggest Inform games, written over a decade apart, Counterfeit Monkey (2012) and Never Gives Up Her Dead (2023) – both had slowdown where a big part of the fix was just narrowing the circumstances in which heavy pathfinding and relations/regex code (respectively) was run.
No, that’s in all things. But that part from the Inform handbook – I think the repercussions of it are mostly gone due to increased efficiency in Inform itself, and maybe the endless speed up of computers. In other words, it’s true that all Before, Instead and After rules run are checked every single turn, but the checks now go so fast it makes no difference.
@Dannii ran some test on this because I was harping on it for years. I can never find the darn outcome when I need to cite it. But that was the outcome.
I think @mathbrush is typing as I’m typing. You can look at his Never Gives Up Her Dead thread to see some real life work on speeding up his huge game just last year. Debugging game slowdowns
-Wade