luceos my xenforo forum on namecheap $15/year basic steller hosting with 1gb memory can handle 50+ online without failure. I use opcache, cloudflare with page rules. I also somewhat limited the cronjob while maintaining all the functionality.
I tried to flood the website with bots (for test), the online counter hitted 700+ online before the resource usage limited error message came (or something like that came) .. usually my website has 30-50 online most of the time, xenforo handel it like pro! Also the design I'm using is top-notch.
While my flarum test website on the same hosting, loaded very slow with no one visiting except me. I also installed some basic FOF plugins example, upload, Google login, seo , etc . Like 4-5 plugins. Website load time was 5-8 seconds. My xenforo website loads within 2 seconds and on load 3-4 seconds.
I Guess this is the difference of quality of free and paid solutions.