That's a very good question and it might be worth to write a post about it :-).

The speed level determines two thing in a battle:

• the sequence in which the cards play. From fastest to slowest.
• the capacity to be missed by attacks.

The more there is a speed difference between two cards, the more there is a chance the slower one misses the faster one. So if you have the Pealacor Conjurer in the last spot, as protection against sneak attacks, the more speed it has, the more often it will be missed by a less fast attacker. Combined with the flying ability, there are litterally tons of missed attacks. Therefore, as you say this card is perfect to be placed in last position.