That is not completely true. Especially on a first load, Cloudflare can speed up the Javascript static asset loading. And that is exactly one of the things that Google looks at to decide if your site is performant enough to avoid a search engine penalty. The benefits are larger when you have a marketing kind of forum that attracts users from search engines, and the downsides are larger when you have a forum with an active, returning, group of users.
No, because it really depends on whether those users are very active or whether they visit once a day. I’ve seen a group of < 50 people exchanging large photo’s and using discourse as a chat box for the entire day bring really performant hardware on its knees, while I have also seen a forum with 10,000 people that came by to post something once a week and over 30 million (!) lurkers run on a mediocre VPS.