Hi, I noticed that every time I paste a medium long or long text (more than 2000 chars) in the Preview Text Tool to work with space and kerning Glyphs gets incredibly slow. This only happens in Mojave, in specific iMac iMac (Retina 5K, 27-inch, Late 2015) and some MacBook Pro 13 inch, late 2018 models.
The preview is not meant for such large amounts of text, because the font preview at the bottom of Edit view is a live-rendered font, which is pretty processor intensive. If you want to try big amounts of text, consider the free SILE plug-in (Window > Plugin Manager), the free PreviewText app (in the Tools section of this web page), or InDesign via the Adobe Fonts folder. Or simply close the Preview area.