I meant those who specialize in magic.

My apologies, esteemed colleague, I misunderstood. ;)
But: 1) not everyone has such a specialization.
2) for those who do have such a specialization, in order to use it, they need to cast the acceleration spell, while Mullich immediately adds +2 to speed.
Under the same conditions, Mullich casts acceleration first (because his creatures are initially faster), and after his turn, there may not even be anyone left to accelerate on the opponent's side. :p

If we consider all heroes, then, for example, the Dragon Gnolls will last until they reach the battlefield. Or will Mullich's units run and switch places to get hit by the Gnolls?

I'm not very sure that the main problem in the enemy army will be the Gnolls. :p IMHO, of course, but I think many will agree.
And again, I repeat, Mullich's specialization gives a great chance to go first, and that sometimes means a lot.