I have restarted the browser and pc. I can’t remember how fast it was before. At least, it is now much faster and more browsable without pain.
But still a bit sluggish compared to SO for example. Scrolling SO homepage a few times longer than here takes half the time.
I can’t say what was the cause for slugging, yet I might add this gave me an another sight on the design. It may still be sluggish on low-core slow-memory machines (mine is 10y old but 8GB with 8-core, so still fast) as not all of us have high resources. It is worth having an optimizing look at the Forum’s codes.