Couple things to check.Payload size, are they different between a logged in user and guest? Which page is this happening on? All pages or just a specific page? Do you have any custom plugins?
Long loading pages categories
I created issue on the github. Please, it is very important, for me.
Comment on the issue:
I've written before about long loading in the administrative panel of categories, but it is important this issue!
If more than 1500 categories, the loading becomes unbearably long, especially on slower servers, more than 10 seconds! On a very good server, it is more than 3 seconds.
Please, deal with it as soon as possible! I do not want to change NodeBB ipboard, phpbb or something.
Please tell me when it will be decided?