not sure exactly what you mean, doesn't seem to be slow for me
When just clicking back and forth and not doing much, the forum needs a lot of time thinking ... I would suggest less thinking and faster switch between pages 
But when i actually do something, as posting this text, it uses less time. At least it feels like it
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I tried to record it to show what I mean
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It's the same with Chrome and Edge. But when I'm logged out - it's fast
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Thats very odd, mine is pretty much instant if I hit the back button in the browser.
For info, the browsers back button works independantly of any websites tbh, the browser itself deals with fetching pages from it's history and may or may not call the server when doing so (it should pull them from your local cache).
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It's the same on my mobile btw