Related: https://developer.blender.org/T57936

And also, perhaps a reference: https://devtalk.blender.org/t/framerate-drops-to-1fps-or-less-with-a-few-hundred-thousand-faces-selected/5067

(And note that instancing currently exaggerates the problem exponentially, last time I checked.)

These module tasks are for technical information about how to solve the problem.

Please don't use developer.blender.org as a discussion forum, that's what https://devtalk.blender.org/c/user-feedback is for.