Not true at all. Even small, insular, just-a-few-hundred users PHPBB-style forums have to run Cloudflare or Anubis to try to stem DDoS-style scraping, and have a constant patrol of moderators to stop AI spam posts.
Not true. You can also make your code fast enough so your server doesn't get overloaded by three requests per second. Then you can just handle the requests.
There are multiple facets to the problem. Technical site performance is a very small part of those.
The disruption, manipulation, and most of all erosion of trust which bad-faith actors create (AI or otherwise) are the most serious issues in my experience. The concept of "evaporative cooling", in which a platform starting to go bad sees an accelerated departure of well-intentioned, high-value participants.
I agree with you that the PHP forum software is not fast at all. But, try hundreds of requests per second, including requests that require text searches.
Also, uh, how does speeding it up that help with the AI spam? (Sorry, I should have emphasized that part of my most more, since that's mostly the topic of this sub-thread I was replying to.)