I have added recent widgets through dynamic widgets plugin to display only on activity page they work but that page loads slow, i then enabled debug and it show some errors at the top of recent posts widget also i have a plugin which show all errors then it fix automatically if they have a fix that is a paid plugin, i just wanted to show you the errors and you maybe tell if that is normal or not. I think Activity page slowness is after last wpForo update not sure as it was loading fast as other pages.
I see no errors in what you posted. I only see notices that are probably harmless.
Ok i have to correct a little on Recent page of wpForo i use to show.
Recent Posts Display Type > Posts
If i select to display Topics that page loads normal as the others if i put back to display Posts it slows down even without widgets so not sure why cause all was fine before from what i remember.
With this i can exclude widgets and those notices problem is something else.
You just need to edit the Recent Topics widgets, select the Ordering type using the new option and save it in Dashboard > Appearance > Widgets admin page.
I found out that Recent page is slow caused by custom avatars something related to images possible because they are too big.
Does wpForo resize them when uploaded or not?