Whenever I click on threads that have a lot of posts, say over 40, the browser goes into "not responding" mode. If I click on the taskbar the browser eventually responds and behaves itself, but it can take up to a couple of minutes.
Any ideas as to why, or what to do? I'm currently using IE 7 but this also happened with IE 6.
Your browser might be choking on old cached files. Try purging the cache first. Then try throttling back on amount of cache memory the browser thinks it needs to use.
In IE6's menu bar: Tools / Internet Options / General / Temporary Internet Files. Hit the Delete files to clear out the cache.
Then set the cache smaller by doing two things. Tools / Internet Options / General / History / Temporary Internet Files / Settings / Amount of disk space to use. The amount of memory specified varies, but I typically set mine to about 64MB. OK out of everything, close out of IE, and restart IE.
Lastly: you can also reduce IE's cache load by not making it remember 20 days worth of surfing history. That's the default setting. I set mine to 0 days, and clear the history while I'm there. You may choose not to go that drastic, but I find cutting it back helps browser lag.