Just to let you know, I’m currently looking at this.
When the output log isn’t being modified, most of the perf. drop comes from text rendering. I’ve done some work to improve the font cache access, but really there’s some larger Slate perf. changes that will become part of 4.7 that should make this equivalent of the perf. in 4.4.
When the output log is being modified, as in the case of your “log once per-frame” case, the perf. drop comes from the increasing layout complexity as new text is added. Seeing as we’re not wrapping the output log, there should be some nice wins that we can get by skipping some parts of the layout process. I’m currently investigating these.