Strange and serious performance issue in forums

I have a severe performance issue with the forums. Here is some info
- If I sign out this does not occur
- Affects the list of topics in a single forum
- Does not affect the list of forums, or the display of a single topic, or repies
- There is a variable delay from a few seconds to several minutes. If it takes too long the browser may time out
- Seen on multiple networks, systems and browsers
- Using tabs I can browse to show multiple forums, one per tab. All the tabs I use will lock up, and then all of them will unlock together
- Has been happening for several months, but recently became much worse
- Sometimes the problem goes away completely for hours
- I am located in UK

- I have a lot of contributions (over 45,000)

Any thoughts or fixes welcome