Highlight the current prompt line in zsh

If you're happy with highlighting only the prompt, then the following will apply reverse colors on whatever you have already in PS1:

PS1="\[\e[7m\]$PS1\[\e(B\e[m\]"

If you have a space at the end of the prompt (this is common practice) that you don't want to highlight:

PS1="[\e[7m]${PS1::-1}[\e(B\e[m] "

Some terminals, at least iTerm for macOS, have the option to set a "cursor guide". This is probably doing exactly what you want to achieve (I have exaggerated the guide in the picture, I usually have it much more transparent, and red, as in the second picture):

The cursor guide has nothing to do with the shell and will be always be displayed, regardless of what you're doing (very faint red cursor guide on the line reading AC_CONF_AUX_DIR([build]), here shown in the vim editor):

Unfortunately I only run iTerm on macOS, so I don't know if there are terminal emulators out there that can do this for your particular Unix.

I don't know what oh-my-zsh puts in the prompt by default. Maybe it tries to identify the version control status, that's a very popular prompt component which might be time-consuming.

To see what's going on, turn on command traces with set -x.

→ ~ 
→ ~ set -x
trace of the commands that are executed to calculate the prompt
→ ~ 
trace of the commands that are executed to calculate the prompt
→ ~ set +x
+zsh:3> set +x
→ ~ 
→ ~ 

If the trace is so long that it scrolls off the screen, redirect it to a file with

exec 2>zsh.err

This directs all error messages to the file, not just the trace. To get traces and errors back on the terminal, run

exec 2>/dev/tty

You can customize the trace format through PS4. This is a format string which can contain prompt escapes. For example, to add precise timing information:

PS4='%D{%s.%9.}+%N:%i> '