When the metal renderer is enabled the screen updates at 60fps. When it is not enabled, the refresh rate is governed by the current bandwidth. There are cases where the new renderer has lower throughput because it does not use adaptive framerate. I’ll look into this.

As another datapoint: for a 100k line log file, iterm2 had up to 2.5x variability between runs and the fastest was about twice slower than Terminal.app.

I tried it myself without benchmarking and it felt much faster, but actually timing it shows the original renderer was about 1.5s faster. Fascinating. I'm guessing the original renderer drops frames.