I have always had terrible performance problems when working with scaled images. If I load a small image into a glyph and scale it up to fit there is noticeable lag when moving points. Glyphs has always been like this and I’ve just never got around to reporting it. I know I can scale the images up in Photoshop before loading them into Gyphs but I find it faster to just drop the images into Glyphs and scale them.
Could you send me a .glyphs file and the image that I can check your specific setup?
I have the same issue, with any image in different projects. I don’t think there is anything image or font specific. I have the latest cutting edge and the latest os, but it has been like that since I first tried working with images in Glyphs (not long ago, actually).
The slowness happens when I zoom in, without even scaling the image.
I sent the file.