Levo,
If you think about it if you had a list of items so tagged, when you tag something else it need to be updated to include the new item.
This immediate update and always "up to date" nature is a key usability aspect of TiddlyWiki. Any change can be reflected anywhere it is referenced (in view) It takes time but you do learn to understand it, that the consequences of one change, must be reflected in what you can see on the screen or it would be out of date, so the less on the screen, the less there is to update. This is normally not an issue, but in some cases this reduces performance because you are asking it to do more.
by closing tiddlers you also reduce the items listed in the side bar open tab etc... every relevant change is also reflected in various tabs in the sidebar, so the side bar can be updated with a change as well. That is why hiding it may also help.
Computers makes doing things very easy, this means we tend to ask it to do more without even thinking about what we are asking it to do, we just expect it.
Regards
Tony