Well, the issue is that the cache system is not well managed by wpForo. Maybe you have to think about letting pro cache plugins manage cache politics. i.e, Wp rocket, for example.
My blog rocks with Cloudflare and WProcket, and the blog page is heavier (image, script, CSS, etc.) than the forum page.
I did the test with this site https://wpforo.com/community/ and on mobile, the KPIs are not very good too.
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Checking my forum home, mobile, with pagespeed insights, i got a 30 speed index.
Disabling my adsense ads plugin totally, i went to 61 speed index. Go figure how reliable the speed meter is
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Google penalize speed but slowdown come from his Adsense script/code/api. I don't use analytics, but imagine if i did. I would get a 10 speed metering.