Slow has never meant majestic. This is more about a war between Ben Jonson and William Shakespeare than about the actual Greek hero Ajax.
Ajax is nowhere in Homer's Iliad described as dull witted. The grandson of Zeus, he is very large (his shield is so large it is likened to a wall, and is covered with seven cow-hides). He fights Hector - Troy's greatest warrior - one-on-one twice but is not defeated.) He is brave, fights very valiantly, and is the most important warrior the Achaeans (Greeks) have after Achilles. However, after Achilles' death, it is Odysseus, not Ajax, who is awarded the magical armor, a decision which eventually causes the death of Ajax.
Shakespeare, however, when writing his play, was in a pretty fierce battle of words and wit with another great, Ben Jonson.
PBS.org's In Search of Shakespeare: 1599: The War of the Poets
The rise in popularity of the Boys' Companies is instrumental in a rift between Ben Jonson and William Shakespeare. The spat takes in other poets and playwrights supportive of the Boys' Companies and their detractors and also opens a rift between purveyors of classically based works (Jonson) and more populist works (Shakespeare). Tit-for-tat hostilities ensue, and volleys of witty insults and torturous puns are unleashed in a Helm's Deep style battle for linguistic dominance.
No one dies, but some language is badly mangled.
One of the ways he took Jonson down was with his pen. He turned Ajax into Ben Jonson with numerous references to him in Troilus and Cressida (and some other plays) and then lampoons him again and again, the following being only one of the many negative references. Shakespeare writes:
...he is as valiant as the lion, churlish as the bear, slow as the elephant: a man into whom nature hath so crowded humours that his valour is crushed into folly, his folly sauced with discretion: there is no man hath a virtue that he hath not a glimpse of, nor any man an attaint but he carries some stain of it...
In Shakespeare and the Poet's War by James P. Bednarz, the author traces the history of the creative conflicts between Shakespeare and Ben Jonson:
Bednarz redraws the Poets' War as a debate on the social function of drama and the status of the dramatist... He shows how this controversy, triggered by Jonson's bold new dramatic experiments, directly influenced the writing of As You Like It, Twelfth Night, Troilus and Cressida, and Hamlet, gave rise to the first modern drama criticism in English, and shaped the way we still perceive Shakespeare today.
Perhaps the Italian translator didn't understand (or didn't agree) that this was a vandalized portrayal of Ajax, and, familiar with the Iliad, chose to portray the well-known stature of Ajax (majestic in size, as an elephant) instead of slow as an elephant.
Some translators do take small liberties with their material. Certainly Shakespeare did worse than that with Homer's heroes of The Iliad.