Sforno on Exodus 4:10/ספורנו על שמות ד׳:י׳

1לא איש דברים אנכי; I am not experienced in knowing how to address people in authority, such as kings.

2גם מתמול, גם משלשום, neither when I was a stranger in a foreign land, nor when I grew up in Pharaoh’s palace, nor since You have spoken to me and I have responded.

5כי כבד פה וכבד לשון אנכי, the reason why I do not possess this facility is because the physical tools with which to express my thoughts are defective. I have therefore never even made the effort to develop this facility (compare Isaiah 50,4) לדעת לעות את יעף דבר, “to know how to speak timely words to the weary.”