According to Mathworks: "it is a reported bug for releases before release R2008b. The issue is it can be slow when unpacking logged timeseries data from a ModelDataLogs object or using the time series tool for timeseries with many events. This effect is only noticeable the first time the timeseries are unpacked. And it typically occurs when the model that generated it contains an enabled subsystem which experiences many enable/disable events.
We have fixed this problem from release of R2008b, and I am afraid that the best solution for this problem is to upgrade your MATLAB to a newer release."
Oh well, at least I know now.