Hard to say without knowing some specifics.  Are the slow workbooks large
ones with lots of calculations to be done?  Perhaps some macro code taking
place to really tie things up.

Have you tried setting Calculation to Manual (in Tools | Options on the
[Calculations] tab) and seeing if that doesn't speed things up?

I wouldn't call 'not responding' typical, but it has been known to happen
with Excel and other applications when they get very busy or are trying to do
something and are waiting for something else, such as data to be retrieved
over a network.