What Exactly Was Moshe's Speech Impediment?

Our Sedra records Moshe describing himself as having "sealed (‘uncircumcised’) lips." Earlier, he had said he was "heavy of mouth, heavy of tongue." We know the famous Medrash that the angel Gavriel moved baby Moshe’s hand to grab a glowing coal, after which he put his hand in his mouth, burning it. But what was the exact nature of his defect? There are many possibilities: He stuttered; he talked with a lisp; he could not pronounce certain letters that used either the tongue (e.g. lamed, tav) or the lips (e.g. pey, mem). Others say he had forgotten his Egyptian during the 80 years he spent in Midian, & so couldn’t properly address Paro. Or perhaps it was his Hebrew that was deficient , since he was raised in Paro’s palace and may have been more comfortable in Egyptian than Hebrew (many sources say Moshe was ordered by Hashem to use only Lashon HaKodesh, the Holy Language, when dealing with Pharoah). Thus he said, "If the Israelites don’t understand (my Hebrew), then how will Paro? Or perhaps he spoke too fast, or he was not glib (i.e. he was "tongue-tied). While by the end of the Torah, Moshe has indeed become a great orator, this may teach us that public speaking is not necessarily the #1 prerequisite for being a great leader!