Interesting read, never considered iTerm to be slow and I'm so grateful it exists but maybe I'm not as hardcore a terminal user as I thought I was :).
Also recent versions include a metal renderer[3] (not sure if it's enabled by default) which might be even faster, I have been running it for some time but have not noticed much difference.
Another reason why it might be hard to notice is also because when one is using ssh you anyways have the network latency so might be one is simply trained not to react to latency in terminals as much.