Both are horribly slow, seriously, try iTerm2 for a week then try Terminal - you'll then notice how much latency iTerm2 adds to basic key presses.
oh-my-zsh is also horribly slow out of box, spend some time manually configuring zsh and observe your terminals open in fractional seconds instead of multiple seconds.
oh-my-zsh is also horribly slow out of box, spend some time manually configuring zsh and observe your terminals open in fractional seconds instead of multiple seconds.