In Terminal, I can cat a 1.5M line file in about 5 seconds, it takes about twice as long in the new iTerm beta.
But iTerm is certainly updating the screen more often than Terminal (it looks smoother at least), so maybe that's because Terminal is doing a better job of allowing the screen buffer to update independently of the drawing thread.
It's a minor thing, but may be something to consider.
Occasionally, but less frequently, you can run into a similar symptoms just running build scripts.
"Important: It is highly recommended that you run the make.sh script in either a very fast terminal such as xterm (the GNOME terminal and the OS X terminal are too slow) or that you run it in a detached GNU screen session (use C-a d to detach the session and screen -r to resume it). The SBCL compile produces lots of output and your system's terminal program will likely slow down the compile in a drastic manner."
I've had this handle since around 1998, but other people started grabbing it in other forums after that. I'm not the twitter ninkendo nor the steam ninkendo either.
I only have this handle without trailing numbers on here, Ars Technica's forums, and gmail. (Formerly reddit too, but I deleted my account there when the site became a bit too toxic.)