I’ve noticed that, when I load a lot of text into the edit window for spacing, the preview window stops showing the selected part of the text, when scrolling down past a certain number of glyphs. It’d just show the beginning of the text. Is the number of glyphs displayed in the preview window limited?
If you want to test long texts, consider using the Adobe Fonts folder:
http://www.glyphsapp.com/tutorials/testing-your-fonts-in-adobe-apps
The preview is not built for long texts because it calculates an intermediate OTF on the fly. Lots of text slows it down significantly.