SignalTap II takes forever to export VCD file

I captured some signals with SignalTap. Since I would like to see the waveforms with better display, I need to export to a VCD file. However it takes forever to complete. I came back the next morning and saw it was completed.

Sometimes it was not too bad. But when I have many signals changing almost per sample, I have the above problem. It looks that Altera did the software in a very inefficient way.