None of this is mine, I found it posted on a website.
Magi characters have at least relativistic reaction speed and I will prove how.
First I'll prove that the magic in Magi is indeed natural phenomena and not magical attacks.
This page states that ruhk creates all natural phenomena in the world and magicians manipulate the ruhk to create magic.
http://m.*********.co/manga/magi/v05/c046/15.html
this page also states Judal "created thunder" http://m.*********.co/manga/magi/v05/c046/16.html
Another statement that proves ruhk creates all natural phenomena in the world. http://m.*********.co/manga/magi/v12/c151/8.html
Miss Myers literally says that they're creating natural calamities with magic. http://m.*********.co/manga/magi/v12/c151/9.html
Myers again states that magicians create natural phenomena
http://i.imgur.com/0Fibfv2.png
Now that I've provided statements of magic being natural, I'll get onto the feats that prove that the magic is natural phenomena.
Aladdin used magic to manipulate heat and water to create a mirage http://m.*********.co/manga/magi/v06/c103/3.html
More proof of magic behaving like their natural counter parts. http://m.*********.co/manga/magi/v06/c109/8.html
The light was dispersed/weakened by Yam's fog.
Yam reflected the light attack back using a mirror, reflection is another property of light. http://m.*********.co/manga/magi/v06/c109/16.html http://m.*********.co/manga/magi/v06/c109/17.html
Aladdin's light ray was also weakened by steam magic. http://m.*********.co/manga/magi/v12/c150/12.html
Now that I've proven that magic attacks behave like natural phenomena I'll get onto the light speed reaction feats.
Titus reacted to Aladdin's light ray http://m.*********.co/manga/magi/v12/c150/12.html
Yam reacted to several light speed attacks http://m.*********.co/manga/magi/v06/c109/14.html
http://m.*********.co/manga/magi/v06/c109/16.html
http://m.*********.co/manga/magi/v06/c109/2.html
And Elder David has also reacted to light speed attacks
http://www1.mangafreak.net/Read1_Magi_230_8
That's three different characters that have reacted to light speed attacks on three different occasions so it isn't an outlier either.
Since they are only reacting to the light attacks and not dodging them I'll leave their speed at relativistic.