I have a rather large Excel file (around 100 MB) and it is very slow when I make changes to it or save it. The task manager shows that my CPU usage is upwards toward 100%. It says that "Excel cannot complete this task with available resources".
The resources problems in Excel are well known and documented. The cause unfortunately is not so easy to pinpoint depending on the various combinations of features which are being used. One simple thing worth trying is to set the formula recalculations to manual to avoid the possibility of entering into a very long chain of events each time a change is made.
You can also try disabling 3rd party add-ons from loading with Excel, before you try that I would try Excel Safe Mode first (hold down CTRL key while launching Excel).
Has anyone else noticed, that we seem to get more and more of installed harmful programs? It's not some cookies, it's stuff that gets installed? And they don't necessarily show in Control Panel, or Revo - they only have their own uninstall... comes the day when they don't even have that? Don't get any less gray hair... blaah.