You  kinda  answered  the  question  yourself,  the  answer  is  speed.  Each  champ  had  a  turn  meter  and  how  quickly  the  turn  meter  is  filled,  outside  of  turnmeter  boosting  abilities,  is  dependant  on  speed.  If  your  champ  has  very  low  speed  compared  to  the  opposing  champs,  lets  say  they  have  twice  the  speed  of  your  champ,  then  they  will  go  twice  before  your  champ  goes  once  because  their  tirn  meter  will  he  filling  up  teoce  as  fast.  

Yeah, so a character with 201 speed will get his second turn just before a character with 100 speed will get his first. But there are many skills, auras, masteries,... which can give a turn meter boost to a champion or your entire team, as well as some skills that can reduce turn meter on opponents (though that requires an accuracy check). So when your opponent is boosting his own team's TM *and* reducing yours, it's possible that the entire team will take two, three or even more turns before you get even one. 

Excuse my language but this sucks!

It's not fair for a turn based game!

I mean i try to figure out all the statistics to make some good champions and here comes someone that boosts only speed, plays two consecutive times in row and poof there goes my team...

That's NOT FAIR

in Arena its the whole point of speed teams to kill the other before they get a turn, as they are usually glass cannons who loose if the other party survives

you have 2 options

either be faster

or make a team thats beefy enough to survive the inital onslaught 

(by using shield sets, immune sets against def down debuff, lots hp/defence, hp/def aura leads, etc etc.)

for newer players the first option is the easier one as the 2nd requires more and better gear/characters/resources

It  makes  the  game  more  dynamic,  were  it  a  simple  turn  based  game  then  it  wouldnt  be  a  popular  as  it  is  now.  A  note  on  the  fairness....  wait  till  you  start  pulling  shards....  then  you're  gonna  find  out  just  how  cruel  and  frustrating  this  game  can  be  😀

No, it's not unfair. There are a lot of champs that can increase the turnmeter of your team. One of them, High Khatun, is given to everybody as login-reward at day 30. Some players just refuse to use her (or another champ with that ability) AND don't come along with a good defebsive strategy either. They just put any 4 champs, let's say attack champs with high damage, into arena and think they could win like that. It's a bad strategy. Losing because of having a bad strategy is not unfair.

The game is about strategy. Your champs have to work together.

Easy example of a speed-team with free acces champs: High Khatun (login-reward, gives speed-boost), Spirithost (can be found in campaigne act 4, gives atk-boost), Warmaiden (can be found in campaign act 9, decreases the def of the whole enemy team), Starterchamp (obviously free to get, deals a lot of AoE damage). That is a good team for the start. Every player could make a team like this.