If the issue is that text is being typed too quickly, you can just add a Set Action Delay action to adjust how much time passes between each character being typed:
How big is the text? I use Insert Text by Typing in Mail’s compose window (in non-Rich Text mode) all the time without issues.
For large text, you are better off using Pasting - it will still remove styles unless you use Insert Styled Text by Pasting (or in this case, you can just use Filter Text: Remove Styles, Command-V.
Why are you wanting to type the clipboard into the Compose window?